Electrolyte solution and method for producing sulfate salt

ABSTRACT

An electrolyte solution capable of providing electrochemical devices whose internal resistance is less likely to increase even after repeated charge and discharge and whose cycle capacity retention ratio is high. The electrolyte solution contains a solvent, an electrolyte salt, and at least one selected from the group consisting of compounds represented by R 11 X 11 —SO 3 M 11  and compounds represented by R 21 R 22 N—SO 3 M 21  in an amount of 0.001 to 10 mass % relative to the solvent.

TECHNICAL FIELD

The present invention relates to electrolyte solutions, andelectrochemical devices, secondary batteries, and modules including theelectrolyte solutions. The present invention also relates to a novelmethod for producing sulfate salts.

BACKGROUND ART

Patent Literature 1 discloses an electrolyte solution for non-aqueouselectrolyte solution secondary batteries that include a positiveelectrode, a negative electrode, and an electrolyte solution in contactwith both of the electrodes, containing an electrolyte, a surfactant,and a phosphazene compound in an aprotic solvent.

However, this literature does not disclose use of an organosulfate thatdoes not serve as a surfactant.

CITATION LIST Patent Literature

-   Patent Literature 1: JP 2014-2972 A

Non-Patent Literature

-   Non-Patent Literature 1: Matthias Schmeisser, and 5 others,    “Coordination of 1,10-phenanthroline and 2,2′-bipyridine to Li+ in    different ionic liquids. How innocent are ionic liquids?”, Inorganic    Chemistry, American Chemical Society, Jun. 13, 2011, 50(14), pp.    6685-6695

SUMMARY OF INVENTION Technical Problem

Improvement of the output characteristics of electrochemical devicessuch as lithium ion secondary batteries requires reduction in theinternal resistance of the electrochemical devices. However,electrochemical devices tend to have a higher internal resistance afterrepeated charge and discharge.

The present invention then aims to provide a novel electrolyte solutioncapable of providing electrochemical devices whose internal resistanceis less likely to increase even after repeated charge and discharge andwhose cycle capacity retention ratio is high.

The present invention also aims to provide a method for producing asulfate salt having a high purity and a low moisture content in highyield.

Solution to Problem

The present invention relates to an electrolyte solution containing: asolvent; an electrolyte salt; and at least one selected from the groupconsisting of compounds represented by the following formula (1) andcompounds represented by the following formula (2) in an amount of 0.001to 10 mass % relative to the solvent,

the formula (1) being R¹¹X¹¹—SO₃M¹¹

wherein R¹¹ is a C1-C6 linear or branched alkyl group, a C2-C6 linear orbranched alkenyl group, a C2-C6 linear or branched alkynyl group, aC3-C6 cycloalkyl group, a C3-C6 cycloalkenyl group, or a C3-C6alkylsilyl group, the alkyl group, the cycloalkyl group, and thealkylsilyl group may have a halogen atom which substitutes for ahydrogen atom bonding to a carbon atom, may have a cyclic structure, andmay have an ether bond or a thioether bond; X¹¹ is O or S; and M¹¹ is atleast one selected from the group consisting of Li, Na, K, and Cs, and

the formula (2) being R²¹R²²N—SO₃M²¹

wherein R²¹ and R²² may be the same as or different from each other, andare individually a C1-C6 linear or branched alkyl group, a C2-C6 linearor branched alkenyl group, a C2-C6 linear or branched alkynyl group, aC3-C6 cycloalkyl group, a C3-C6 cycloalkenyl group, or a C3-C6alkylsilyl group, the alkyl group, the cycloalkyl group, and thealkylsilyl group may have a halogen atom which substitutes for ahydrogen atom bonding to a carbon atom, may have a cyclic structure, andR¹¹ and R²² may bond to each other to form a cyclic structure; and M²¹is at least one selected from the group consisting of Li, Na, K, and Cs.

In the formula (1), preferably, R¹¹ is a C1-C6 linear or branched alkylgroup or a C3-C6 cycloalkyl group; X¹¹ is O; and M¹¹ is Li.

In the formula (2), preferably, R²¹ and R²² may be the same as ordifferent from each other, and are individually a C1-C6 linear orbranched alkyl group or a C3-C6 cycloalkyl group; and M²¹ is Li.

The solvent preferably contains at least one selected from the groupconsisting of non-fluorinated saturated cyclic carbonates, fluorinatedsaturated cyclic carbonates, non-fluorinated acyclic carbonates, andfluorinated acyclic carbonates.

The electrolyte salt is preferably at least one lithium salt selectedfrom the group consisting of LiPF₆, LiBF₄, LiSO₃CF₃, LiN(SO₂CF₃)₂,LiN(SO₂C₂F₅)₂, lithium difluoro(oxalato)borate, lithiumbis(oxalato)borate, and salts represented byLiPF_(a)(C_(n)F_(2n+1))_(6−a) where a is an integer of 0 to 5; and n isan integer of 1 to 6.

The present invention also relates to an electrochemical deviceincluding the above electrolyte solution.

The present invention also relates to a secondary battery including theabove electrolyte solution.

The present invention also relates to a module including the aboveelectrochemical device or secondary battery.

The present invention also relates to a method for producing a compoundrepresented by the following formula (1), including: reacting a compoundrepresented by the following formula (3) and a metal alkoxide,

the formula (1) being R¹¹X¹¹—SO₃M¹¹

wherein R¹¹ is a C1-C6 linear or branched alkyl group, a C2-C6 linear orbranched alkenyl group, a C2-C6 linear or branched alkynyl group, aC3-C6 cycloalkyl group, a C3-C6 cycloalkenyl group, or a C3-C6alkylsilyl group, the alkyl group, the cycloalkyl group, and thealkylsilyl group may have a halogen atom which substitutes for ahydrogen atom bonding to a carbon atom, may have a cyclic structure, andmay have an ether bond or a thioether bond; X¹¹ is O; and M¹¹ is atleast one selected from the group consisting of Li, Na, K, and Cs, and

the formula (3) being R³¹O—SO₂—OR³²

wherein R³¹ and R³² may be the same as or different from each other, andare individually a C1-C6 linear or branched alkyl group, a C2-C6 linearor branched alkenyl group, a C2-C6 linear or branched alkynyl group, aC3-C6 cycloalkyl group, a C3-C6 cycloalkenyl group, or a C3-C6alkylsilyl group, the alkyl group, the cycloalkyl group, and thealkylsilyl group may have a halogen atom which substitutes for ahydrogen atom bonding to a carbon atom, may have a cyclic structure, andmay have an ether bond or a thioether bond.

In the formula (1), preferably, R¹¹ is a C1-C6 linear or branched alkylgroup or a C3-C6 cycloalkyl group; and M¹¹ is Li.

In the formula (3), preferably, R³¹ and R³² may be the same as ordifferent from each other, and are individually a C1-C6 linear orbranched alkyl group or a C3-C6 cycloalkyl group.

The metal alkoxide is preferably a lithium alkoxide, more preferably atleast one selected from the group consisting of lithium methoxide andlithium ethoxide.

The production method of the present invention preferably furtherincludes reacting an alcohol with lithium metal to provide a lithiumalkoxide.

In the production method of the present invention, the reaction ispreferably caused by adding 0.7 to 1.5 mol of the metal alkoxiderelative to 1 mol of the compound represented by the formula (3).

Advantageous Effects of Invention

Utilizing the electrolyte solution of the present invention enablesproduction of electrochemical devices whose internal resistance is lesslikely to increase even after repeated charge and discharge and whosecycle capacity retention ratio is high.

The production method of the present invention enables production of asulfate salt having a high purity and a low moisture content in highyield.

DESCRIPTION OF EMBODIMENTS

The present invention will be specifically described hereinbelow.

The electrolyte solution of the present invention contains a solvent, anelectrolyte salt, and at least one selected from the group consisting ofcompounds represented by the formula (1) and compounds represented bythe formula (2) in an amount of 0.001 to 10 mass % relative to thesolvent.

The compounds represented by the formula (1) are represented by theformula (1):

R¹¹X¹¹—SO₃M¹¹

wherein R¹¹ is a C1-C6 linear or branched alkyl group, a C2-C6 linear orbranched alkenyl group, a C2-C6 linear or branched alkynyl group, aC3-C6 cycloalkyl group, a C3-C6 cycloalkenyl group, or a C3-C6alkylsilyl group, the alkyl group, the cycloalkyl group, and thealkylsilyl group may have a halogen atom which substitutes for ahydrogen atom bonding to a carbon atom, may have a cyclic structure, andmay have an ether bond or a thioether bond; X¹¹ is O or S; and M¹¹ is atleast one selected from the group consisting of Li, Na, K, and Cs.

In the formula (1), R¹¹ is preferably a C1-C6 linear or branched alkylgroup or a C3-C6 cycloalkyl group, more preferably a C1-C6 linear orbranched alkyl group. The alkyl group or the cycloalkyl group preferablydoes not have a halogen atom which substitutes for a hydrogen atombonding to a carbon atom, does not have a cyclic structure, and does nothave an ether bond or a thioether bond. The carbon number of the alkylgroup is more preferably 5 or less, still more preferably 4 or less,while preferably 1 or greater, more preferably 2 or greater.

In the formula (1), R¹¹ is preferably at least one selected from thegroup consisting of a methyl group, an ethyl group, a propyl group, anisopropyl group, a butyl group, an isobutyl group, a t-butyl group, ahexyl group, a 2-fluoroethyl group, and a 2,2,2-trifluoroethyl group,more preferably a methyl group, an ethyl group, a propyl group, anisopropyl group, a butyl group, a hexyl group, and a2,2,2-trifluoroethyl group, still more preferably at least one selectedfrom the group consisting of a methyl group, an ethyl group, a propylgroup, an isopropyl group, a butyl group, and a 2,2,2-trifluoroethylgroup, particularly preferably at least one selected from the groupconsisting of an ethyl group, a propyl group, an isopropyl group, and abutyl group, further more preferably an ethyl group.

In the formula (1), X¹¹ is O or S, preferably O.

In the formula (1), M¹¹ is at least one selected from the groupconsisting of Li, Na, K, and Cs, preferably Li.

Examples of the compounds represented by the formula (1) includeCH₃OSO₃Li, C₂H₅OSO₃Li, CH₃CH₂CH₂OSO₃Li, CH₃CH(CH₃)OSO₃Li,CH₃CH₂CH₂CH₂OSO₃Li, CH₃CH₂CH(CH₃)OSO₃Li, CH₃CH₂C(CH₃)₂OSO₃Li,CH₃CH₂CH₂CH₂CH₂CH₂OSO₃Li, CFH₂CH₂OSO₃Li, and CF₃CH₂OSO₃Li.

In order to provide electrochemical devices whose internal resistance ismuch less likely to increase even after repeated charge and dischargeand whose cycle capacitance retention ratio is higher, in the formula(1), preferably, R¹¹ is a C1-C6 linear or branched alkyl group or aC3-C6 cycloalkyl group, X¹¹ is O, and M¹¹ is Li. In other words, thecompounds represented by the formula (1) are preferably lithiummonoalkyl sulfates represented by the formula (1-1):

R¹¹O—SO₃Li

wherein R¹¹ is a C1-C6 linear or branched alkyl group or a C3-C6cycloalkyl group, the alkyl group may have a halogen atom whichsubstitutes for a hydrogen atom bonding to a carbon atom, and may have acyclic structure, and the cycloalkyl group may have a halogen atom whichsubstitutes for a hydrogen atom bonding to a carbon atom.

Preferred as the compounds represented by the formula (1) is at leastone selected from the group consisting of CH₃OSO₃Li, C₂H₅OSO₃Li,CH₃CH₂CH₂OSO₃Li, CH₃CH(CH₃)OSO₃Li, CH₃CH₂CH₂CH₂OSO₃Li,CH₃CH₂CH(CH₃)OSO₃Li, CH₃CH₂C(CH₃)₂OSO₃Li, CH₃CH₂CH₂CH₂CH₂CH₂OSO₃Li,CHF₂CH₂OSO₃Li, and CF₃CH₂OSO₃Li, more preferred is at least one selectedfrom the group consisting of C₂H₅OSO₃Li, CH₃CH₂CH₂OSO₃Li,CH₃CH(CH₃)OSO₃Li, and CH₃CH₂CH₂CH₂OSO₃Li, still more preferred isC₂H₅OSO₃Li.

The compounds represented by the formula (2) are represented by theformula (2):

R²¹R²²N—SO₃M²¹

wherein R²¹ and R²² may be the same as or different from each other, andare individually a C1-C6 linear or branched alkyl group, a C2-C6 linearor branched alkenyl group, a C2-C6 linear or branched alkynyl group, aC3-C6 cycloalkyl group, a C3-C6 cycloalkenyl group, or a C3-C6alkylsilyl group, the alkyl group, the cycloalkyl group, and thealkylsilyl group may have a halogen atom which substitutes for ahydrogen atom bonding to a carbon atom, and may have a cyclic structure,and R²¹ and R²² may bond to each other to form a cyclic structure; andM²¹ is at least one selected from the group consisting of Li, Na, K, andCs.

In the formula (2), preferably, R²¹ and R²² may be the same as ordifferent from each other, and are individually a C1-C6 linear orbranched alkyl group or a C3-C6 cycloalkyl group, and M²¹ is Li. Thealkyl group or the cycloalkyl group preferably does not have a halogenatom which substitutes for a hydrogen atom bonding to a carbon atom,does not have a cyclic structure, and does not have an ether bond or athioether bond.

In the formula (2), M²¹ is at least one selected from the groupconsisting of Li, Na, K, and Cs, preferably Li.

Examples of the compounds represented by the formula (2) include(CH₃)₂NSO₃Li, (C₂H₅)₂NSO₃Li, (CH₃)(C₂H₅)NSO₃Li, (C₃H₇)₂NSO₃Li,(C₃H₇)(CH₃)NSO₃Li, (C₃H₇)(C₂H₅)NSO₃Li, (C₄H₉)₂NSO₃Li, and(C₅H₁₁)₂NSO₃Li. More preferred as the compounds represented by theformula (2) is at least one selected from the group consisting of(CH₃)₂NSO₃Li and (C2H₅)₂NSO₃Li.

The electrolyte solution of the present invention contains 0.001 to 10mass % of at least one selected from the group consisting of thecompounds represented by the formula (1) and the compounds representedby the formula (2) relative to the solvent. The amount of thesecompounds is preferably 0.001 mass % or more, more preferably 0.05 mass% or more, while preferably 10 mass % or less, more preferably 5 mass %or less.

The electrolyte solution of the present invention preferably contains,as at least one selected from the group consisting of the compoundsrepresented by the formula (1) and the compounds represented by theformula (2), a lithium monoalkyl sulfate represented by the formula(1-1):

R¹²O—SO₃Li

wherein R¹² is a C1-C6 linear or branched alkyl group or a C3-C6cycloalkyl group, the alkyl group may have a halogen atom whichsubstitutes for a hydrogen atom bonding to a carbon atom, and may have acyclic structure, and the cycloalkyl group may have a halogen atom whichsubstitutes for a hydrogen atom bonding to a carbon atom.

In the formula (1-1), R¹² is preferably a C1-C6 linear or branched alkylgroup having neither a halogen atom which substitutes for a hydrogenatom bonding to a carbon atom nor a cyclic structure. The carbon numberof the alkyl group is more preferably 5 or less, still more preferably 4or less, while preferably 1 or greater, more preferably 2 or greater.

In the formula (1-1), R¹² is preferably at least one selected from thegroup consisting of a methyl group, an ethyl group, a propyl group, anisopropyl group, a butyl group, an isobutyl group, a t-butyl group, ahexyl group, a 2-fluoroethyl group, and a 2,2,2-trifluoroethyl group,more preferably a methyl group, an ethyl group, a propyl group, anisopropyl group, a butyl group, a hexyl group, and a2,2,2-trifluoroethyl group, still more preferably at least one selectedfrom the group consisting of a methyl group, an ethyl group, a propylgroup, an isopropyl group, a butyl group, and a 2,2,2-trifluoroethylgroup, particularly preferably at least one selected from the groupconsisting of an ethyl group, a propyl group, an isopropyl group, and abutyl group, further more preferably an ethyl group.

The lithium monoalkyl sulfate is preferably at least one selected fromthe group consisting of CH₃OSO₃Li, C₂H₅OSO₃Li, CH₃CH₂CH₂OSO₃Li,CH₃CH(CH₃)OSO₃Li, CH₃CH₂CH₂CH₂OSO₃Li, CH₃CH₂CH(CH₃)OSO₃Li,CH₃CH₂C(CH₃)₂OSO₃Li, CH₃CH₂CH₂CH₂CH₂CH₂OSO₃Li, CFH₂CH₂OSO₃Li, andCF₃CH₂OSO₃Li, more preferably at least one selected from the groupconsisting of C₂H₅OSO₃Li, CH₃CH₂CH₂OSO₃Li, CH₃CH(CH₃)OSO₃Li, andCH₃CH₂CH₂CH₂OSO₃Li, still more preferably C₂H₅OSO₃Li.

If the electrolyte solution of the present invention contains a lithiummonoalkyl sulfate represented by the formula (1-1) as at least oneselected from the group consisting of the compounds represented by theformula (1) and the compounds represented by the formula (2), the amountof the lithium monoalkyl sulfate is 0.001 to 10 mass % relative to thesolvent. The amount of the lithium monoalkyl sulfate is preferably 0.001mass % or more, more preferably 0.05 mass % or more, while preferably 10mass % or less, more preferably 5 mass % or less.

The electrolyte solution of the present invention contains a solvent.The solvent is preferably a non-aqueous solvent and the electrolytesolution of the present invention is preferably a non-aqueouselectrolyte solution.

The solvent preferably contains at least one selected from the groupconsisting of fluorinated acyclic carbonates, non-fluorinated saturatedcyclic carbonates, fluorinated saturated cyclic carbonates, andnon-fluorinated acyclic carbonates.

The fluorinated acyclic carbonate is a chain carbonate containing afluorine atom.

The fluorinated acyclic carbonate preferably has a fluorine content of10 to 70.0 mass %. The fluorine content may be calculated based on thestructural formula of the fluorinated acyclic carbonate by

{(number of fluorine atoms×19)/(molecular weight of fluorinated acycliccarbonate)}×100(%).

Examples of the fluorinated acyclic carbonate include fluorinatedacyclic carbonates represented by the formula: Rf¹OCOORf² (where Rf¹ andRf² are the same as or different from each other and are individually aC1-C4 alkyl group or fluorine-containing alkyl group, but at least oneof Rf¹ and Rf² is a C1-C4 fluorine-containing alkyl group.

Rf¹ and Rf² are the same as or different from each other and areindividually a C1-C4 alkyl group or a C1-C4 fluorine-containing alkylgroup, but at least one of Rf¹ and Rf² is a C1-C4 fluorine-containingalkyl group.

The above carbon number is preferably 1 to 3 in order to achieve goodcompatibility with the electrolyte solution.

Examples of Rf¹ include CF₃—, CF₃CF₂—, (CF₃)₂CH—, CF₃CH₂—, C₂F₅CH₂—,HCF₂CH₂—, HCF₂CF₂CH₂—, and CF₃CFHCF₂CH₂—. Preferred among these areCF₃CH₂— and HCF₂CH₂— in terms of high flame retardance, good ratecharacteristics, and good oxidation resistance.

Examples of Rf² include CF₃—, CF₃CF₂—, (CF₃)₂CH—, CF₃CH₂—, C₂F₅CH₂—,HCF₂CH₂—, HCF₂CF₂CH₂—, and CF₃CFHCF₂CH₂—. Preferred among these areCF₃CH₂— and HCF₂CH₂— in terms of high flame retardance, good ratecharacteristics, and good oxidation resistance.

Specific examples of the fluorinated acyclic carbonate includefluorinated acyclic carbonates such as CF₃CH₂OCOOCH₂CF₃, CF₃CH₂OCOOCH₃,CF₃CF₂CH₂OCOOCH₂CF₂CF₃, and CF₃CF₂CH₂OCOOCH₃. Examples thereof furtherinclude compounds disclosed in JP H06-21992 A, JP 2000-327634 A, and JP2001-256983 A. Preferred among these is at least one compound selectedfrom the group consisting of CF₃CH₂OCOOCH₂CF₃, CF₃CH₂OCOOCH₃, andCF₃CF₂CH₂OCOOCH₂CF₂CF₃ in terms of high effectiveness of suppressinggeneration of gas to improve the high-temperature storagecharacteristics. The fluorine content is more preferably 20 mass % ormore, still more preferably 30 mass % or more, particularly preferably33 mass % or more. The fluorine content is more preferably 60 mass % orless, still more preferably 55 mass % or less.

Examples of the non-fluorinated saturated cyclic carbonates includeethylene carbonate (EC), propylene carbonate (PC), and butylenecarbonate.

In order to achieve a high permittivity and a suitable viscosity, thenon-fluorinated saturated cyclic carbonate is preferably at least onecompound selected from the group consisting of ethylene carbonate,propylene carbonate, and butylene carbonate.

The non-fluorinated saturated cyclic carbonate may include one of theabove compounds or may include two or more thereof in combination.

The fluorinated saturated cyclic carbonate is a saturated cycliccarbonate with a fluorine atom attached thereto. Specific examplesthereof include a fluorinated saturated cyclic carbonate (A) representedby the following formula (A):

wherein X¹ to X⁴ may be the same as or different from each other, andare individually a fluorinated alkyl group which may optionally have —H,—CH₃, —F, or an ether bond, or a fluorinated alkoxy group which mayoptionally have an ether bond; at least one of X¹ to X⁴ is a fluorinatedalkyl group which may optionally have —F or an ether bond, or afluorinated alkoxy group which may optionally have an ether bond.

If the electrolyte solution of the present invention contains afluorinated saturated cyclic carbonate (A) and is applied to a lithiumion secondary battery, a stable film is formed on the negative electrodeso that side reactions of the electrolyte solution on the negativeelectrode may sufficiently be suppressed. As a result, significantlystable, excellent charge and discharge characteristics can be achieved.

The term “ether bond” herein means a bond represented by —O—.

In order to achieve a good permittivity and oxidation resistance, one ortwo of X¹ to X⁴ in the formula (A) is/are preferably a fluorinated alkylgroup which may optionally have —F or an ether bond or a fluorinatedalkoxy group which may optionally have an ether bond.

In anticipation of a decrease in the viscosity at low temperatures, anincrease in the flash point, and improvement in the solubility of theelectrolyte salt, X¹ to X⁴ in the formula (A) are preferablyindividually —H, —F, a fluorinated alkyl group (a), a fluorinated alkylgroup (b) having an ether bond, or a fluorinated alkoxy group (c).

The fluorinated alkyl group (a) is an alkyl group in which at least onehydrogen atom is replaced by a fluorine atom. The fluorinated alkylgroup (a) preferably has a carbon number of 1 to 20, more preferably 2to 17, still more preferably 2 to 7, particularly preferably 2 to 5.

If the carbon number is too large, the low-temperature characteristicsmay be poor and the solubility of the electrolyte salt may be low. Ifthe carbon number is too small, the solubility of the electrolyte saltmay be low, the discharge efficiency may be low, and the viscosity maybe high, for example.

Examples of the fluorinated alkyl group (a) which has a carbon number of1 include CFH₂—, CF₂H—, and CF₃—.

In order to achieve a good solubility of the electrolyte salt, preferredexamples of the fluorinated alkyl group (a) which has a carbon number of2 or greater include fluorinated alkyl groups represented by thefollowing formula (a-1):

R¹—R²—  (a-1)

wherein R¹ is an alkyl group which may optionally have a fluorine atomand which has a carbon number of 1 or greater; R² is a C1-C3 alkylenegroup which may optionally have a fluorine atom; and at least one of R¹and R² has a fluorine atom.

R¹ and R² each may further have an atom other than the carbon atom,hydrogen atom, and fluorine atom.

R¹ is an alkyl group which may optionally have a fluorine atom and whichhas a carbon number of 1 or greater. R¹ is preferably a C1-C16 linear orbranched alkyl group. The carbon number of R¹ is more preferably 1 to 6,still more preferably 1 to 3.

Specifically, for example, CH₃—, CH₃CH₂—, CH₃CH₂CH₂—, CH₃CH₂CH₂CH₂—, andthe groups represented by the following formulas:

may be mentioned as linear or branched alkyl groups for R¹.

If R¹ is a linear alkyl group having a fluorine atom, examples thereofinclude CF₃—, CF₃CH₂—, CF₃CF₂—, CF₃CH₂CH₂—, CF₃CF₂CH₂—, CF₃CF₂CF₂—,CF₃CH₂CF₂—, CF₃CH₂CH₂CH₂—, CF₃CF₂CH₂CH₂—, CF₃CH₂CF₂CH₂—, CF₃CF₂CF₂CH₂—,CF₃CF₂CF₂CF₂—, CF₃CF₂CH₂CF₂—, CF₃CH₂CH₂CH₂CH₂—, CF₃CF₂CH₂CH₂CH₂—,CF₃CH₂CF₂CH₂CH₂—, CF₃CF₂CF₂CH₂CH₂—, CF₃CF₂CF₂CF₂CH₂—, CF₃CF₂CH₂CF₂CH₂—,CF₃CF₂CH₂CH₂CH₂CH₂—, CF₃CF₂CF₂CF₂CH₂CH₂—, CF₃CF₂CH₂CF₂CH₂CH₂—, HCF₂—,HCF₂CH₂—, HCF₂CF₂—, HCF₂CH₂CH₂—, HCF₂CF₂CH₂—, HCF₂CH₂CF₂—,HCF₂CF₂CH₂CH₂—, HCF₂CH₂CF₂CH₂—, HCF₂CF₂CF₂CF₂—, HCF₂CF₂CH₂CH₂CH₂—,HCF₂CH₂CF₂CH₂CH₂—, HCF₂CF₂CF₂CF₂CH₂—, HCF₂CF₂CF₂CF₂CH₂CH₂—, FCH₂—,FCH₂CH₂—, FCH₂CF₂—, FCH₂CF₂CH₂—, FCH₂CF₂CF₂—, CH₃CF₂CH₂—, CH₃CF₂CF₂—,CH₃CH₂CH₂—, CH₃CF₂CH₂CF₂—, CH₃CF₂CF₂CF₂—, CH₃CH₂CF₂CF₂—,CH₃CF₂CH₂CF₂CH₂—, CH₃CF₂CF₂CF₂CH₂—, CH₃CF₂CF₂CH₂CH₂—, CH₃CH₂CF₂CF₂CH₂—,CH₃CF₂CH₂CF₂CH₂—, CH₃CF₂CH₂CF₂CH₂CH₂—, CH₃CF₂CH₂CF₂CH₂CH₂—,HCFClCF₂CH₂—, HCF₂CFClCH₂—, HCF₂CFClCF₂CFClCH₂—, andHCFClCF₂CFClCF₂CH₂—.

If R¹ is a branched alkyl group having a fluorine atom, thoserepresented by the following formulas:

may be preferably mentioned. If the group has a branch represented by—CH₃ or —CF₃, for example, the viscosity is likely to be high. Thus, thenumber of such branches is more preferably small (one) or zero.

R² is a C1-C3 alkylene group which may optionally have a fluorine atom.R² may be a linear or branched group. Examples of a minimum structuralunit constituting such a linear or branched alkylene group are shownbelow. R² is constituted by one or combination of these units.

(i) Linear Minimum Structural Units

—CH₂—, —CHF—, —CF₂—, —CHCl—, —CFCl—, —CCl₂—(ii) Branched minimum structural units

Preferred among these exemplified units are Cl-free structural unitsbecause such units are not dehydrochlorinated by a base, and thus aremore stable.

If R² is a linear group, the group consists only of the above linearminimum structural unit, preferably —CH₂—, —CH₂CH₂—, or —CF₂—. In orderto further improve the solubility of the electrolyte salt, —CH₂— or—CH₂CH₂— is more preferred.

If R² is a branched group, the group includes at least one of the abovebranched minimum structural units. Preferred examples thereof includethose represented by the formula: —(CX^(a)X^(b))— (wherein X^(a) is H,F, CH₃, or CF₃; X^(b) is CH₃ or CF₃; if X^(b) is CF₃, X^(a) is H orCH₃). Such groups can further improve the solubility of the electrolytesalt.

For example, CF₃CF₂—, HCF₂CF₂—, H₂CFCF₂—, CH₃CF₂—, CF₃CF₂CF₂—,HCF₂CF₂CF₂—, H₂CFCF₂CF₂—, CH₃CF₂CF₂—, and those represented by thefollowing formulas:

may be mentioned as preferred fluorinated alkyl groups (a).

The fluorinated alkyl group (b) having an ether bond is an alkyl groupwhich has an ether bond and in which at least one hydrogen atom isreplaced by a fluorine atom. The fluorinated alkyl group (b) having anether bond preferably has a carbon number of 2 to 17. If the carbonnumber is too large, the fluorinated saturated cyclic carbonate (A) mayhave a high viscosity, and also the number of fluorine-containing groupsincreases. Thus, the solubility of the electrolyte salt may be poor dueto reduction in the permittivity, and the compatibility with othersolvents may be poor. Accordingly, the carbon number of the fluorinatedalkyl group (b) having an ether bond is preferably 2 to 10, morepreferably 2 to 7.

The alkylene group which constitutes the ether segment of thefluorinated alkyl group (b) having an ether bond may be a linear orbranched alkylene group. Examples of a minimum structural unitconstituting such a linear or branched alkylene group are shown below.

(i) Linear Minimum Structural Units

—CH₂—, —CHF—, —CF₂—, —CHCl—, —CFCl—, —CCl₂—

(ii) Branched Minimum Structural Units

The alkylene group may be constituted by one of these minimum structuralunits alone, or may be constituted by a combination of linear units (i),of branched units (ii), or of a linear unit (i) and a branched unit(ii). Preferred examples will be mentioned in detail later.

Preferred among these exemplified units are Cl-free structural unitsbecause such units are not dehydrochlorinated by a base, and thus aremore stable.

Still more preferred examples of the fluorinated alkyl group (b) havingan ether bond include those represented by the following formula (b-1):

R³—(OR⁴)_(n1)—  (b-1)

wherein R³ is preferably a C1-C6 alkyl group which may optionally have afluorine atom; R⁴ is preferably a C1-C4 alkylene group which mayoptionally have a fluorine atom; n1 is an integer of 1 to 3; and atleast one of R³ and R⁴ has a fluorine atom.

Examples of the groups for R³ and R⁴ include the following, and anyappropriate combination of these groups can provide the fluorinatedalkyl group (b) having an ether bond represented by the formula (b-1).Still, the groups are not limited thereto.

(1) R³ is preferably an alkyl group represented by the formula: X^(c)₃C—(R⁵)_(n2)—, where three X^(c)'s may be the same as or different fromeach other, and are individually H or F; R⁵ is a C1-C5 alkylene groupwhich may optionally have a fluorine atom; and n2 is 0 or 1.

If n2 is 0, R³ may be CH₃—, CF₃—, HCF₂—, or H₂CF—, for example.

If n2 is 1, specific examples of a linear group for R³ include CF₃CH₂—,CF₃CF₂—, CF₃CH₂CH₂—, CF₃CF₂CH₂—, CF₃CF₂CF₂—, CF₃CH₂CF₂—, CF₃CH₂CH₂CH₂—,CF₃CF₂CH₂CH₂—, CF₃CH₂CF₂CH₂—, CF₃CF₂CF₂CH₂—, CF₃CF₂CF₂CF₂—,CF₃CF₂CH₂CF₂—, CF₃CH₂CH₂CH₂CH₂—, CF₃CF₂CH₂CH₂CH₂—, CF₃CH₂CF₂CH₂CH₂—,CF₃CF₂CF₂CH₂CH₂—, CF₃CF₂CF₂CF₂CH₂—, CF₃CF₂CH₂CF₂CH₂—,CF₃CF₂CH₂CH₂CH₂CH₂—, CF₃CF₂CF₂CF₂CH₂CH₂—, CF₃CF₂CH₂CF₂CH₂CH₂—, HCF₂CH₂—,HCF₂CF₂—, HCF₂CH₂CH₂—, HCF₂CF₂CH₂—, HCF₂CH₂CF₂—, HCF₂CF₂CH₂CH₂—,HCF₂CH₂CF₂CH₂—, HCF₂CF₂CF₂CF₂—, HCF₂CF₂CH₂CH₂CH₂—, HCF₂CH₂CF₂CH₂CH₂—,HCF₂CF₂CF₂CF₂CH₂—, HCF₂CF₂CF₂CF₂CH₂CH₂—, FCH₂CH₂—, FCH₂CF₂—,FCH₂CF₂CH₂—, FCH₂CF₂CH₂—, CH₃CF₂—, CH₃CH₂—, CH₃CF₂CH₂—, CH₃CF₂CF₂—,CH₃CH₂CH₂—, CH₃CF₂CH₂CF₂—, CH₃CF₂CF₂CF₂—, CH₃CH₂CF₂CF₂—, CH₃CH₂CH₂CH₂—,CH₃CF₂CH₂CF₂CH₂—, CH₃CF₂CF₂CF₂CH₂—, CH₃CF₂CF₂CH₂CH₂—, CH₃CH₂CF₂CF₂CH₂—,CH₃CH₂CF₂CF₂CH₂CH₂—, and CH₃CF₂CH₂CF₂CH₂CH₂—.

If n2 is 1, those represented by the following formulas:

may be mentioned as branched groups for R³.

If the group for R³ has a branch such as —CH₃ or —CF₃, the viscosity islikely to be high. Thus, the group for R³ is more preferably a lineargroup.

(2) In the segment —(OR⁴)_(n1)— of the formula (b-1), n1 is an integerof 1 to 3, preferably 1 or 2. If n1 is 2 or 3, R⁴'s may be the same asor different from each other.

Preferred specific examples of the group for R⁴ include the followinglinear or branched groups.

Examples of the linear groups include —CH₂—, —CHF—, —CF₂—, —CH₂CH₂—,—CF₂CH₂—, —CF₂CF₂—, —CH₂CF₂—, —CH₂CH₂CH₂—, —CH₂CH₂CF₂—, —CH₂CF₂CH₂—,—CH₂CF₂CF₂—, —CF₂CH₂CH₂—, —CF₂CF₂CH₂—, —CF₂CH₂CF₂—, and —CF₂CF₂CF₂—.

Those represented by the following formulas:

may be mentioned as branched groups.

The fluorinated alkoxy group (c) is an alkoxy group in which at leastone hydrogen atom is replaced by a fluorine atom. The fluorinated alkoxygroup (c) preferably has a carbon number of 1 to 17. The carbon numberis more preferably 1 to 6.

The fluorinated alkoxy group (c) is particularly preferably afluorinated alkoxy group represented by the formula: X^(d) ₃C—(R⁶)_(n3)—O— (wherein three X^(d)'s may be the same as or different fromeach other, and are individually H or F; R⁶ is preferably a C1-C5alkylene group which may optionally have a fluorine atom; n3 is 0 or 1;and any one of the three X^(d)'s is a fluorine atom).

Specific examples of the fluorinated alkoxy group (c) includefluorinated alkoxy groups in which an oxygen atom is bonded to an end ofthe alkyl group for R¹ in the formula (a-1).

The fluorinated alkyl group (a), the fluorinated alkyl group (b) havingan ether bond, and the fluorinated alkoxy group (c) in the fluorinatedsaturated cyclic carbonate (A) each preferably have a fluorine contentof 10 mass % or more. If the fluorine content is too low, an effect ofincreasing the flash point may not be sufficiently achieved. Thus, thefluorine content is more preferably 20 mass % or more, still morepreferably 30 mass % or more. The upper limit thereof is usually 85 mass%.

The fluorine content of each of the fluorinated alkyl group (a), thefluorinated alkyl group (b) having an ether bond, and the fluorinatedalkoxy group (c) is a value calculated by the following formula:

{(Number of fluorine atoms×19)/(formula weight of the formula)}×100(%)

based on the corresponding structural formula.

In order to achieve a good permittivity and oxidation resistance, thefluorine content in the whole fluorinated saturated cyclic carbonate (A)is preferably 5 mass % or more, more preferably 10 mass % or more. Theupper limit thereof is usually 76 mass %.

The fluorine content in the whole fluorinated saturated cyclic carbonate(A) is a value calculated based on the structural formula of thefluorinated saturated cyclic carbonate (A) by the following formula:

{(Number of fluorine atoms×19)/(molecular weight of fluorinatedsaturated cyclic carbonate (A))}×100(%).

Specific examples of the fluorinated saturated cyclic carbonate (A)include the following.

Those represented by the following formulas:

may be mentioned as specific examples of the fluorinated saturatedcyclic carbonate (A) represented by the formula (A) in which at leastone of X¹ to X⁴ is —F. These compounds have a high withstand voltage andgive a good solubility of the electrolyte salt.

Alternatively, those represented by the following formula:

may also be used.

Those represented by the following formulas:

may be mentioned as specific examples of the fluorinated saturatedcyclic carbonate (A) represented by the formula (A) in which at leastone of X¹ to X⁴ is a fluorinated alkyl group (a) and the others thereofare —H.

Those represented by the following formulas:

may be mentioned as specific examples of the fluorinated saturatedcyclic carbonate (A) represented by the formula (A) in which at leastone of X¹ to X⁴ is a fluorinated alkyl group (b) having an ether bond ora fluorinated alkoxy group (c) and the others thereof are —H.

The fluorinated saturated cyclic carbonate (A) is not limited to theabove specific examples. One of the above fluorinated saturated cycliccarbonates (A) may be used alone, or two or more thereof may be used inany combination at any ratio. Preferred amounts of the fluorinatedsaturated cyclic carbonates will be mentioned later, and such preferredamounts correspond to preferred amounts of the fluorinated saturatedcyclic carbonates (A).

Preferred as the fluorinated saturated cyclic carbonate (A) arefluoroethylene carbonate and difluoroethylene carbonate.

Examples of the non-fluorinated acyclic carbonate include hydrocarbonacyclic carbonates such as CH₃OCOOCH₃ (dimethyl carbonate: DMC),CH₃CH₂OCOOCH₂CH₃ (diethyl carbonate: DEC), CH₃CH₂OCOOCH₃ (ethyl methylcarbonate: EMC), CH₃OCOOCH₂CH₂CH₃ (methyl propyl carbonate), methylbutyl carbonate, ethyl propyl carbonate, and ethyl butyl carbonate.Preferred among these is at least one compound selected from the groupconsisting of dimethyl carbonate, ethyl methyl carbonate, diethylcarbonate, methyl propyl carbonate, methyl butyl carbonate, ethyl propylcarbonate, and ethyl butyl carbonate.

The electrolyte solution of the present invention preferably contains 10to 99.99 mass %, more preferably 10 to 95 mass %, still more preferably15 to 90 mass %, of the solvent relative to the electrolyte solution.

The solvent preferably contains 40 to 100 vol %, more preferably 60 to100 vol %, still more preferably 90 to 100 vol %, particularlypreferably 100 vol %, in total of the non-fluorinated saturated cycliccarbonate, fluorinated saturated cyclic carbonate, non-fluorinatedacyclic carbonate, and fluorinated acyclic carbonate.

The solvent preferably contains at least one saturated cyclic carbonateselected from the group consisting of non-fluorinated saturated cycliccarbonates and fluorinated saturated cyclic carbonates and at least oneacyclic carbonate selected from the group consisting of non-fluorinatedacyclic carbonates and fluorinated acyclic carbonates.

The volume ratio of the saturated cyclic carbonate and the acycliccarbonate is preferably 10/90 to 90/10, more preferably 30/70 or higher,while more preferably 70/30 or lower.

The solvent preferably contains a non-fluorinated acyclic carbonate. Thenon-fluorinated acyclic carbonate is preferably in an amount of 30 vol %or more, more preferably 40 vol % or more, still more preferably 50 vol% or more, while preferably 90 vol % or less, more preferably 80 vol %or less, relative to the solvent. The electrolyte solution whichcontains a solvent containing a non-fluorinated acyclic carbonate can besuitably used for electrochemical devices used at relatively lowvoltages.

If the solvent contains a non-fluorinated acyclic carbonate, the solventpreferably further contains at least one saturated cyclic carbonateselected from the group consisting of non-fluorinated saturated cycliccarbonates and fluorinated saturated cyclic carbonates.

The solvent preferably contains 70 to 100 vol %, more preferably 80 to100 vol %, still more preferably 90 to 100 vol %, particularlypreferably 100 vol %, in total of the non-fluorinated acyclic carbonateand the saturated cyclic carbonate.

The volume ratio of the non-fluorinated acyclic carbonate and thesaturated cyclic carbonate is preferably 10/90 to 95/5, more preferably30/70 or higher, still more preferably 40/60 or higher, while morepreferably 90/10 or lower.

If the solvent contains both the non-fluorinated saturated cycliccarbonate and the fluorinated saturated cyclic carbonate as the cycliccarbonates, the volume ratio of the non-fluorinated saturated cycliccarbonate and the fluorinated saturated cyclic carbonate is preferably10/90 to 90/10, preferably 30/70 or higher, while preferably 70/30 orlower.

The solvent preferably contains a fluorinated acyclic carbonate. Thefluorinated acyclic carbonate is preferably in an amount of 20 vol % ormore, more preferably 30 vol % or more, while preferably 90 vol % orless, more preferably 80 vol % or less, relative to the solvent. Theelectrolyte solution which contains a solvent containing a fluorinatedacyclic carbonate can be suitably used for electrochemical devices to beused at relatively high voltages.

If the solvent contains a fluorinated acyclic carbonate alone as theacyclic carbonate, the fluorinated acyclic carbonate is preferably in anamount of 50 vol % or more, more preferably 60 vol % or more, whilepreferably 90 vol % or less, more preferably 80 vol % or less, relativeto the solvent.

If the solvent contains a fluorinated acyclic carbonate, the solventalso preferably further contains a non-fluorinated acyclic carbonate.

The solvent preferably contains 50 vol % or more, more preferably 60 vol% or more, while preferably 90 vol % or less, more preferably 80 vol %or less, in total of the fluorinated acyclic carbonate and thenon-fluorinated acyclic carbonate.

The volume ratio of the fluorinated acyclic carbonate and thenon-fluorinated acyclic carbonate is preferably 5/95 or higher,preferably 10/90 or higher, while preferably 50/50 or lower, morepreferably 40/60 or lower.

If the solvent contains a fluorinated acyclic carbonate, the solventpreferably further contains at least one saturated cyclic carbonateselected from the group consisting of non-fluorinated saturated cycliccarbonates and fluorinated saturated cyclic carbonates.

The solvent preferably contains 40 to 100 vol %, more preferably 60 to100 vol %, still more preferably 90 to 100 vol %, particularlypreferably 100 vol %, in total of the fluorinated acyclic carbonate andthe saturated cyclic carbonate.

The volume ratio of the non-fluorinated acyclic carbonate and thesaturated cyclic carbonate is preferably 10/90 to 95/5, more preferably30/70 or higher, more preferably 40/60 or higher, while more preferably90/10 or lower.

If the solvent contains both the non-fluorinated saturated cycliccarbonate and the fluorinated saturated cyclic carbonate as the cycliccarbonates, the volume ratio of the non-fluorinated saturated cycliccarbonate and the fluorinated saturated cyclic carbonate is preferably10/90 to 90/10, more preferably 30/70 or higher, while more preferably70/30 or lower.

The electrolyte solution of the present invention contains anelectrolyte salt.

Any electrolyte salt usable for electrolyte solutions forelectrochemical devices such as secondary batteries and electricdouble-layer capacitors may be used. Preferred is a lithium salt.

Examples of the lithium salt include inorganic lithium salts such asLiClO₄, LiPF₆, and LiBF₄; and fluoroorganic acid lithium salts such asLiSO₃CF₃, LiN(SO₂CF₃)₂, LiN(SO₂C₂F₅)₂, LiN(SO₂CF₃)(SO₂C₄F₉),LiC(SO₂CF₃)₃, LiPF₄(CF₃)₂, LiPF₄(C₂F₅)₂, LiPF₄(SO₂CF₃)₂,LiPF₄(SO₂C₂F₅)₂, LiBF₂(CF₃)₂, LiBF₂(C₂F₅)₂, LiBF₂(SO₂CF₃)₂,LiBF₂(SO₂C₂F₅)₂, lithium difluoro(oxalato)borate, lithiumbis(oxalato)borate, and salts represented by the formula:LiPF_(a)(C_(n)F_(2n+1))_(6−a) (wherein a is an integer of 0 to 5; and nis an integer of 1 to 6). These may be used alone or in combination oftwo or more.

In order to suppress degradation of the electrolyte solution afterhigh-temperature storage, the lithium salt is particularly preferably atleast one selected from the group consisting of LiPF₆, LiBF₄, LiSO₃CF₃,LiN(SO₂CF₃)₂, LiN(SO₂C₂F₅)₂, lithium difluoro(oxalato)borate, lithiumbis(oxalato)borate, and salts represented by the formula:LiPF_(a)(C_(n)F_(2n+1))_(6−a) (wherein a is an integer of 0 to 5; and nis an integer of 1 to 6).

Examples of the salts represented by the formula:LiPF_(a)(C_(n)F_(2n+1))_(6−a) include LiPF₃(CF₃)₃, LiPF₃(C₂F₅)₃,LiPF₃(C₃F₇)₃, LiPF₃(C₄F₉)₃, LiPF₄(CF₃)₂, LiPF₄(C₂F₅)₂, LiPF₄(C₃F₇)₂, andLiPF₄(C₄F₉)₂ wherein the alkyl group represented by C₃F₇ or C₄F₉ in theformula may be either linear or branched.

The concentration of the electrolyte salt in the electrolyte solution ispreferably 0.5 to 3 mol/L. If the concentration thereof is outside thisrange, the electrolyte solution tends to have a low electricconductivity and the battery performance tends to be impaired.

The concentration of the electrolyte salt is more preferably 0.9 mol/Lor more and 1.5 mol/L or less.

The electrolyte salt in the electrolyte solution for electric doublelayer capacitors is preferably an ammonium salt.

Examples of the ammonium salt include the following salts (IIa) to(IIe).

(IIa) Tetraalkyl Quaternary Ammonium Salts

Preferred examples thereof include tetraalkyl quaternary ammonium saltsrepresented by the following formula (IIa):

(wherein R^(1a), R^(2a), R^(3a), and R^(4a) may be the same as ordifferent from each other, and are individually a C1-C6 alkyl groupwhich may optionally have an ether bond; and X⁻ is an anion). In orderto improve the oxidation resistance, part or all of the hydrogen atomsin the ammonium salt is/are also preferably replaced by a fluorine atomand/or a C1-C4 fluorine-containing alkyl group.

Preferred specific examples of the tetraalkyl quaternary ammonium saltsinclude tetraalkyl quaternary ammonium salts represented by thefollowing formula (IIa-1):

(R^(1a))_(x)(R^(2a))_(y)N^(⊕)X^(⊖)  (IIa-1)

(wherein R^(1a), R^(2a), and X⁻ are defined in the same manner asmentioned above; x and y may be the same as or different from eachother, and are individually an integer of 0 to 4, where x+y=4), andalkyl ether group-containing trialkyl ammonium salts represented by thefollowing formula (IIa-2):

(wherein R^(5a) is a C1-C6 alkyl group; R^(6a) is a C1-C6 divalenthydrocarbon group; R^(7a) is a C1-C4 alkyl group; z is 1 or 2; and X⁻ isan anion). Introduction of an alkyl ether group may lead to reduction inthe viscosity.

The anion X⁻ may be either an inorganic anion or an organic anion.Examples of the inorganic anion include AlCl₄ ⁻, BF₄ ⁻, PF₆ ⁻, AsF₆ ⁻,TaF₆ ⁻, I⁻, and SbF₆ ⁻. Examples of the organic anion include CF₃COO⁻,CF₃SO₃ ⁻, (CF₃SO₂)₂N⁻, and (C₂F₅SO₂)₂N⁻.

In order to achieve good oxidation resistance and ionic dissociation,BF₄ ⁻, PF₆ ⁻, AsF₆ ⁻, and SbF₆ ⁻ are preferred.

Preferred specific examples of the tetraalkyl quaternary ammonium saltsinclude Et₄NBF₄, Et₄NClO₄, Et₄NPF₆, Et₄NAsF₆, Et₄NSbF₆, Et₄NCF₃SO₃,Et₄N(CF₃SO₂)₂N, Et₄NC₄F₉SO₃, Et₃MeNBF₄, Et₃MeNClO₄, Et₃MeNPF₆,Et₃MeNAsF₆, Et₃MeNSbF₆, Et₃MeNCF₃SO₃, Et₃MeN(CF₃SO₂)₂N, Et₃MeNC₄F₉SO₃,and N,N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium salt. Particularlypreferred examples thereof include Et₄NBF₄, Et₄NPF₆, Et₄NSbF₆, Et₄NAsF₆,Et₃MeNBF₄, and an N,N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium salt.

(IIb) Spirocyclic Bipyrrolidinium Salts

Preferred examples thereof include spirocyclic bipyrrolidinium saltsrepresented by the following formula (IIb-1):

(wherein R^(8a) and R^(9a) may be the same as or different from eachother, and are individually a C1-C4 alkyl group; X⁻ is an anion; n1 isan integer of 0 to 5; and n2 is an integer of 0 to 5); spirocyclicbipyrrolidinium salts represented by the following formula (IIb-2):

(wherein R^(10a) and R^(11a) may be the same as or different from eachother, and are individually a C1-C4 alkyl group; X⁻ is an anion; n3 isan integer of 0 to 5; and n4 is an integer of 0 to 5); and spirocyclicbipyrrolidinium salts represented by the following formula (IIb-3):

(wherein R^(12a) and R^(13a) may be the same as or different from eachother, and are individually a C1-C4 alkyl group; X⁻ is an anion; n5 isan integer of 0 to 5; and n6 is an integer of 0 to 5). In order toimprove the oxidation resistance, part or all of the hydrogen atoms inthe spirocyclic bipyrrolidinium salt is/are also preferably replaced bya fluorine atom and/or a C1-C4 fluorine-containing alkyl group.

Preferred specific examples of the anion X⁻ are the same as thosementioned for the salts (IIa). In order to achieve good dissociation anda low internal resistance under high voltage, BF₄ ⁻, PF₆ ⁻, (CF₃SO₂)₂N⁻,or (C₂F₅SO₂)₂N⁻ is particularly preferred.

For example, those represented by the following formulas:

may be mentioned as preferred specific examples of the spirocyclicbipyrrolidinium salts.

These spirocyclic bipyrrolidinium salts are excellent in the solubilityin a solvent, the oxidation resistance, and the ion conductivity.

(IIc) Imidazolium Salts

Preferred examples thereof include imidazolium salts represented by thefollowing formula (IIc):

(wherein R^(14a) and R^(15a) may be the same as or different from eachother, and are individually a C1-C6 alkyl group; and X⁻ is an anion). Inorder to improve the oxidation resistance, part or all of the hydrogenatoms in the imidazolium salt is/are also preferably replaced by afluorine atom and/or a C1-C4 fluorine-containing alkyl group.

Preferred specific examples of the anion X⁻ are the same as thosementioned for the salts (IIa).

For example, one represented by the following formula:

may be mentioned as a preferred specific example of the imidazoliumsalt.

This imidazolium salt is excellent in that it has a low viscosity and agood solubility.

(IId): N-Alkylpyridinium Salts

Preferred examples thereof include N-alkylpyridinium salts representedby the formula (IId):

(wherein R^(16a) is a C1-C6 alkyl group; and X⁻ is an anion). In orderto improve the oxidation resistance, part or all of the hydrogen atomsin the N-alkylpyridinium salt is/are also preferably replaced by afluorine atom and/or a C1-C4 fluorine-containing alkyl group.

Preferred specific examples of the anion X⁻ are the same as thosementioned for the salts (IIa).

For example, those represented by the following formulas:

may be mentioned as preferred specific examples of the N-alkylpyridiniumsalts.

These N-alkylpyridinium salts are excellent in that they have a lowviscosity and a good solubility.

(IIe) N,N-Dialkylpyrrolidinium Salts

Preferred examples thereof include N,N-dialkylpyrrolidinium saltsrepresented by the formula (IIe):

(wherein R^(17a) and R^(18a) may be the same as or different from eachother, and are individually a C1-C6 alkyl group; and X⁻ is an anion). Inorder to improve the oxidation resistance, part or all of the hydrogenatoms in the N,N-dialkylpyrrolidinium salt is/are also preferablyreplaced by a fluorine atom and/or a C1-C4 fluorine-containing alkylgroup.

Preferred specific examples of the anion X⁻ are the same as thosementioned for the salts (IIa).

For example, those represented by the following formulas:

may be mentioned as preferred specific examples of theN,N-dialkylpyrrolidinium salts.

These N,N-dialkylpyrrolidinium salts are excellent in that they have alow viscosity and a good solubility.

Preferred among these ammonium salts are those represented by theformula (IIa), (IIb), or (IIc) because they have a good solubility,oxidation resistance, and ion conductivity. More preferred are thoserepresented by the following formulas:

wherein Me is a methyl group; Et is an ethyl group; and X⁻, x, and y aredefined in the same manner as in the formula (IIa-1).

The electrolyte salt for electric double layer capacitors may be alithium salt. Preferred examples of the lithium salt include LiPF₆,LiBF₄, LiAsF₆, LiSbF₆, and LiN(SO₂C₂H₅)₂.

In order to further increase the capacity, a magnesium salt may be used.Preferred examples of the magnesium salt include Mg(ClO₄)₂ andMg(OOC₂H₅)₂.

If the electrolyte salt is any of the above ammonium salts, theconcentration thereof is preferably 0.6 mol/L or higher. If theconcentration thereof is lower than 0.6 mol/L, not only thelow-temperature characteristics may be poor but also the initialinternal resistance may be high. The concentration of the electrolytesalt is more preferably 0.9 mol/L or higher.

For good low-temperature characteristics, the upper limit of theconcentration is preferably 3.0 mol/L or lower, more preferably 2.0mol/L or lower.

If the ammonium salt is triethyl methyl ammonium tetrafluoroborate(TEMABF₄), the concentration thereof is preferably 0.8 to 1.9 mol/L inorder to achieve excellent low-temperature characteristics.

If the ammonium salt is spirobipyrrolidinium tetrafluoroborate (SBPBF₄),the concentration thereof is preferably 0.7 to 2.0 mol/L.

The electrolyte solution of the present invention preferably furtherincludes polyethylene oxide that has a weight average molecular weightof 2000 to 4000 and has —OH, —OCOOH, or —COOH at an end.

Containing such a compound improves the stability at the interfacesbetween the electrolyte solution and the respective electrodes, and thuscan improve the battery characteristics.

Examples of the polyethylene oxide include polyethylene oxide monool,polyethylene oxide carboxylate, polyethylene oxide diol, polyethyleneoxide dicarboxylate, polyethylene oxide triol, and polyethylene oxidetricarboxylate. These may be used alone or in combination of two ormore.

In order to achieve good battery characteristics, a mixture ofpolyethylene oxide monool and polyethylene oxide diol and a mixture ofpolyethylene oxide carboxylate and polyethylene oxide dicarboxylate arepreferred.

The polyethylene oxide having too small a weight average molecularweight may be easily oxidatively decomposed. The weight averagemolecular weight is more preferably 3000 to 4000.

The weight average molecular weight can be determined in terms ofpolystyrene equivalent by gel permeation chromatography (GPC).

The amount of the polyethylene oxide is preferably 1×10⁻⁶ to 1×10⁻²mol/kg in the electrolyte solution. If the amount of the polyethyleneoxide is too large, the battery characteristics may be poor.

The amount of the polyethylene oxide is more preferably 5×10⁻⁶ mol/kg ormore.

The electrolyte solution of the present invention preferably furthercontains, as an additive, at least one selected from the groupconsisting of unsaturated cyclic carbonates, fluorinated saturatedcyclic carbonates, and cyclic sulfonic acid compounds. Containing thesecompounds suppresses degradation of the battery characteristics.

The unsaturated cyclic carbonate is a cyclic carbonate having anunsaturated bond, i.e., a cyclic carbonate having at least onecarbon-carbon unsaturated bond in the molecule. Specific examplesthereof include vinylene carbonate compounds such as vinylene carbonate,methyl vinylene carbonate, ethyl vinylene carbonate, 4,5-dimethylvinylene carbonate, and 4,5-diethyl vinylene carbonate; and vinylethylene carbonate compounds such as 4-vinyl ethylene carbonate (VEC),4-methyl-4-vinyl ethylene carbonate, 4-ethyl-4-vinyl ethylene carbonate,4-n-propyl-4-vinylene ethylene carbonate, 5-methyl-4-vinyl ethylenecarbonate, 4,4-divinyl ethylene carbonate, 4,5-divinyl ethylenecarbonate, 4,4-dimethyl-5-methylene ethylene carbonate, and4,4-diethyl-5-methylene ethylene carbonate. Preferred among these isvinylene carbonate, 4-vinyl ethylene carbonate, 4-methyl-4-vinylethylene carbonate, or 4,5-divinyl ethylene carbonate, and particularlypreferred is vinylene carbonate or 4-vinyl ethylene carbonate.

The unsaturated cyclic carbonate may have any molecular weight that doesnot significantly deteriorate the effects of the present invention. Themolecular weight is preferably 50 or higher and 250 or lower. Theunsaturated cyclic carbonate having a molecular weight within this rangeis likely to assure its solubility in the electrolyte solution and toenable sufficient achievement of the effects of the present invention.The molecular weight of the unsaturated cyclic carbonate is morepreferably 80 or higher, while more preferably 150 or lower.

The unsaturated cyclic carbonate may also be preferably a fluorinatedunsaturated cyclic carbonate.

The number of fluorine atoms in the fluorinated unsaturated cycliccarbonate may be any number that is 1 or greater. The number of fluorineatoms is usually 6 or smaller, preferably 4 or smaller, most preferably1 or 2.

Examples of the fluorinated unsaturated cyclic carbonate includefluorinated vinylene carbonate derivatives and fluorinated ethylenecarbonate derivatives substituted with a substituent having an aromaticring or a carbon-carbon double bond.

Examples of the fluorinated vinylene carbonate derivatives include4-fluorovinylene carbonate, 4-fluoro-5-methyl vinylene carbonate,4-fluoro-5-phenyl vinylene carbonate, 4-allyl-5-fluorovinylenecarbonate, and 4-fluoro-5-vinyl vinylene carbonate.

Examples of the fluorinated ethylene carbonate derivatives substitutedwith a substituent having an aromatic ring or a carbon-carbon doublebond include 4-fluoro-4-vinyl ethylene carbonate, 4-fluoro-4-allylethylene carbonate, 4-fluoro-5-vinyl ethylene carbonate,4-fluoro-5-allyl ethylene carbonate, 4,4-difluoro-4-vinyl ethylenecarbonate, 4,4-difluoro-4-allyl ethylene carbonate, 4,5-difluoro-4-vinylethylene carbonate, 4,5-difluoro-4-allyl ethylene carbonate,4-fluoro-4,5-divinyl ethylene carbonate, 4-fluoro-4,5-diallyl ethylenecarbonate, 4,5-difluoro-4,5-divinyl ethylene carbonate,4,5-difluoro-4,5-diallyl ethylene carbonate, 4-fluoro-4-phenyl ethylenecarbonate, 4-fluoro-5-phenyl ethylene carbonate, 4,4-difluoro-5-phenylethylene carbonate, and 4,5-difluoro-4-phenyl ethylene carbonate.

The fluorinated unsaturated cyclic carbonate may have any molecularweight that does not significantly deteriorate the effects of thepresent invention. The molecular weight is preferably 50 or higher and500 or lower. The fluorinated unsaturated cyclic carbonate having amolecular weight within this range is likely to assure the solubility ofthe fluorinated unsaturated cyclic carbonate in the electrolyte solutionand to enable sufficient achievement of the effects of the presentinvention.

The unsaturated cyclic carbonates may be used alone or in anycombination of two or more at any ratio.

Examples of the fluorinated saturated cyclic carbonate include compoundsmentioned as examples of the fluorinated saturated cyclic carbonatesusable for the solvent.

Examples of the cyclic sulfonic acid compounds include 1,3-propanesultone, 1,4-butane sultone, 1-fluoro-1,3-propane sultone,2-fluoro-1,3-propane sultone, and 3-fluoro-1,3-propane sultone.

In order to improve the high-temperature characteristics, theelectrolyte solution of the present invention preferably contains1,3-propane sultone and/or 1,4-butane sultone.

If at least one compound selected from the group consisting of theunsaturated cyclic carbonates, fluorinated saturated cyclic carbonates,and the cyclic sulfonic acid compounds is used as an additive, theamount thereof in the electrolyte solution is preferably 0.1 to 10 mass%, more preferably 1 mass % or more, while more preferably 5 mass % orless.

The electrolyte solution of the present invention may further containany other solvents or additives such as cyclic or acyclic carboxylates,ether compounds, nitrogen-containing compounds, boron-containingcompounds, organic silicon-containing compounds, fireproof agents (flameretardants), surfactants, additives for increasing the permittivity,improvers for cycle characteristics and rate characteristics, andovercharge inhibitors, to the extent that the effects of the presentinvention are not impaired.

Examples of the cyclic carboxylates include those having 3 to 12 carbonatoms in total in the structural formula. Specific examples thereofinclude gamma-butyrolactone, gamma-valerolactone, gamma-caprolactone,and epsilon-caprolactone. Particularly preferred is gamma-butyrolactonebecause it can improve the battery characteristics owing to improvementin the degree of dissociation of lithium ions.

In general, the amount of the cyclic carboxylate is preferably 0.1 mass% or more, more preferably 1 mass % or more, in 100 mass % of thesolvent. The cyclic carboxylate in an amount within this range is likelyto improve the electric conductivity of the electrolyte solution, andthus to improve the large-current discharge characteristics ofelectrolyte batteries. The amount of the cyclic carboxylate is alsopreferably 10 mass % or less, more preferably 5 mass % or less. Such anupper limit may allow the electrolyte solution to have a viscositywithin an appropriate range, may make it possible to avoid a reductionin the electric conductivity, may suppress an increase in the resistanceof the negative electrode, and may allow electrolyte batteries to havelarge-current discharge characteristics within a favorable range.

The cyclic carboxylate to be suitably used may be a fluorinated cycliccarboxylate (fluorolactone). Examples of the fluorolactone includefluorolactones represented by the following formula (C):

wherein X¹⁵ to X²⁰ may be the same as or different from each other, andare individually —H, —F, —Cl, —CH₃, or a fluorinated alkyl group; and atleast one of X¹⁵ to X²⁰ is a fluorinated alkyl group.

Examples of the fluorinated alkyl group for X¹⁵ to X²⁰ include —CFH₂,—CF₂H, —CF₃, —CH₂CF₃, —CF₂CF₃, —CH₂CF₂CF₃, and —CF(CF₃)₂. In order toachieve high oxidation resistance and an effect of improving the safety,—CH₂CF₃ and —CH₂CF₂CF₃ are preferred.

One of X¹⁵ to X²⁰ or a plurality thereof may be replaced by —H, —F, —Cl,—CH₃ or a fluorinated alkyl group only when at least one of X¹⁵ to X²⁰is a fluorinated alkyl group. In order to achieve a good solubility ofthe electrolyte salt, the number of substituents is preferably 1 to 3,more preferably 1 or 2.

The substitution may be at any of the above sites in the fluorinatedalkyl group. In order to achieve a good synthesizing yield, thesubstitution site is preferably X¹⁷ and/or X¹⁸. In particular, X¹⁷ orX¹⁸ is preferably a fluorinated alkyl group, especially, —CH₂CF₃ or—CH₂CF₂CF₃. The substituent for X¹⁵ to X²⁰ other than the fluorinatedalkyl group is —H, —F, —Cl, or CH₃. In order to achieve a goodsolubility of the electrolyte salt, —H is preferred.

In addition to those represented by the above formula, the fluorolactonemay also be a fluorolactone represented by the following formula (D):

wherein one of A and B is CX²⁶X²⁷ (where X²⁶ and X²⁷ may be the same asor different from each other, and are individually —H, —F, —Cl, —CF₃,—CH₃, or an alkylene group in which a hydrogen atom may optionally bereplaced by a halogen atom and which may optionally has a hetero atom inthe chain) and the other is an oxygen atom; Rf¹² is a fluorinated alkylgroup or fluorinated alkoxy group which may optionally have an etherbond; X²¹ and X²² may be the same as or different from each other, andare individually —H, —F, —Cl, —CF₃, or CH₃; X²³ to X²⁵ may be the sameas or different from each other, and are individually —H, —F, —Cl, or analkyl group in which a hydrogen atom may optionally be replaced by ahalogen atom and which may optionally contain a hetero atom in thechain; and n=0 or 1.

Preferred examples of the fluorolactone represented by the formula (D)include a 5-membered ring structure represented by the following formula(E):

(wherein A, B, Rf¹², X²¹, X²², and X²³ are defined in the same manner asin the formula (D)) because it is easily synthesized and has goodchemical stability. Further, in relation to the combination of A and B,fluorolactones represented by the following formula (F):

(wherein Rf¹², X²¹, X²², X²³, X²⁶, and X²⁷ are defined in the samemanner as in the formula (D)) and fluorolactones represented by thefollowing formula (G):

(wherein Rf¹², X²¹, X²², X²³, X²⁶, and X²⁷ are defined in the samemanner as in the formula (D)) may be mentioned.

In order to particularly achieve excellent characteristics such as ahigh permittivity and a high withstand voltage, and to improve thecharacteristics of the electrolyte solution in the present invention,for example, to achieve a good solubility of the electrolyte salt and towell reduce the internal resistance, those represented by the followingformulas:

may be mentioned.

Containing a fluorinated cyclic carboxylate leads to effects of, forexample, improving the ion conductivity, improving the safety, andimproving the stability at high temperature.

Examples of the acyclic carboxylate include those having three to sevencarbon atoms in total in the structural formula. Specific examplesthereof include methyl acetate, ethyl acetate, n-propyl acetate,isopropyl acetate, n-butyl acetate, isobutyl acetate, t-butyl acetate,methyl propionate, ethyl propionate, n-propyl propionate, isopropylpropionate, n-butyl propionate, isobutyl propionate, t-butyl propionate,methyl butyrate, ethyl butyrate, n-propyl butyrate, n-propyl butyrate,isopropyl butyrate, methyl isobutyrate, ethyl isobutyrate, n-propylisobutyrate, and isopropyl isobutyrate.

In order to improve the ion conductivity owing to a reduction in theviscosity, preferred are methyl acetate, ethyl acetate, n-propylacetate, n-butyl acetate, methyl propionate, ethyl propionate, n-propylpropionate, isopropyl propionate, methyl butyrate, and ethyl butyrate,for example.

Also, a fluorinated acyclic carboxylate (fluorine-containing ester) mayalso suitably be used. Preferred examples of the fluorine-containingester include fluorinated acyclic carboxylates represented by thefollowing formula (H):

Rf¹⁰COORf¹¹  (H)

(wherein Rf¹⁰ is a C1-C2 fluorinated alkyl group; and Rf¹¹ is a C1-C4fluorinated alkyl group) because they are high in flame retardance andhave good compatibility with other solvents and good oxidationresistance.

Examples of the group for Rf¹⁰ include CF₃—, CF₃CF₂—, HCF₂CF₂—, HCF₂—,CH₃CF₂—, and CF₃CH₂—. In order to achieve good rate characteristics,CF₃— and CF₃CF₂— are particularly preferred.

Examples of the group for Rf¹¹ include —CF₃, —CF₂CF₃, —CH(CF₃)₂,—CH₂CF₃, —CH₂CH₂CF₃, —CH₂CF₂CFHCF₃, —CH₂C₂F₅, —CH₂CF₂CF₂H, —CH₂CH₂C₂F₅,—CH₂CF₂CF₃, and —CH₂CF₂CF₂CF₃. In order to achieve good compatibilitywith other solvents, —CH₂CF₃, —CH(CF₃)₂, —CH₂C₂F₅, and —CH₂CF₂CF₂H areparticularly preferred.

Specifically, for example, the fluorinated acyclic carboxylate mayinclude one or two or more of CF₃C(═O)OCH₂CF₃, CF₃C(═O)OCH₂CH₂CF₃,CF₃C(═O)OCH₂C₂F₅, CF₃C(═O)OCH₂CF₂CF₂H, and CF₃C(═O)OCH(CF₃)₂. In orderto achieve good compatibility with other solvents and good ratecharacteristics, CF₃C(═O)OCH₂C₂F₅, CF₃C(═O)OCH₂CF₂CF₂H, CF₃C(═O)OCH₂CF₃,and CF₃C(═O)OCH(CF₃)₂ are particularly preferred.

The ether compound is preferably a C3-C10 acyclic ether or a C3-C6cyclic ether.

Examples of the C3-C10 acyclic ether include diethyl ether, di-n-butylether, dimethoxy methane, methoxy ethoxy methane, diethoxy methane,dimethoxy ethane, methoxy ethoxy ethane, diethoxy ethane, ethyleneglycol di-n-propyl ether, ethylene glycol di-n-butyl ether, anddiethylene glycol dimethyl ether.

The ether compound may suitably be a fluorinated ether.

One example of the fluorinated ether is a fluorinated ether (I)represented by the following formula (I):

Rf¹³—O—Rf¹⁴  (I)

(wherein Rf¹³ and Rf¹⁴ may be the same as or different from each other,and are individually a C1-C10 alkyl group or a C1-C10 fluorinated alkylgroup; and at least one of Rf¹³ and Rf¹⁴ is a fluorinated alkyl group).Containing the fluorinated ether (I) can improve the flame retardance ofthe electrolyte solution, as well as improve the stability and safety athigh temperature under high voltage.

In the formula (I), at least one of Rf¹³ and Rf¹⁴ has only to be aC1-C10 fluorinated alkyl group. In order to further improve the flameretardance and the stability and safety at high temperature under highvoltage of the electrolyte solution, both Rf¹³ and Rf¹⁴ are preferably aC1-C10 fluorinated alkyl group. In this case, Rf¹³ and Rf¹⁴ may be thesame as or different from each other.

Preferably, Rf¹³ and Rf¹⁴ are the same as or different from each other,and Rf¹³ is a C3-C6 fluorinated alkyl group and Rf¹⁴ is a C2-C6fluorinated alkyl group.

If the sum of the carbon numbers of Rf¹³ and Rf¹⁴ is too small, thefluorinated ether may have too low a boiling point. If the carbon numberof Rf¹³ or Rf¹⁴ is too large, the solubility of the electrolyte salt maybe low, which may cause a bad influence on the compatibility with othersolvent, and the viscosity may be high so that the rate characteristics(viscosity) may be poor. In order to achieve excellent ratecharacteristics and boiling point, advantageously, the carbon number ofRf¹³ is 3 or 4 and the carbon number of Rf¹⁴ is 2 or 3.

The fluorinated ether (I) preferably has a fluorine content of 40 to 75mass %. The fluorinated ether (I) having a fluorine content within thisrange may lead to particularly excellent balance between the flameretardance and the compatibility. The above range is also preferred forgood oxidation resistance and safety.

The lower limit of the fluorine content is more preferably 45 mass %,still more preferably 50 mass %, particularly preferably 55 mass %. Theupper limit thereof is more preferably 70 mass %, still more preferably66 mass %.

The fluorine content of the fluorinated ether (I) is a value calculatedbased on the structural formula of the fluorinated ether (I) by thefollowing formula:

{(number of fluorine atoms×19)/(molecular weight of fluorinatedether(I))}×100(%).

Examples of the group for Rf¹³ include CF₃CF₂CH₂—, CF₃CFHCF₂—,HCF₂CF₂CF₂—, HCF₂CF₂CH₂—, CF₃CF₂CH₂CH₂—, CF₃CFHCF₂CH₂—, HCF₂CF₂CF₂CF₂—,HCF₂CF₂CF₂CH₂—, HCF₂CF₂CH₂CH₂—, and HCF₂CF(CF₃)CH₂—. Examples of thegroup for Rf¹⁴ include —CH₂CF₂CF₃, —CF₂CFHCF₃, —CF₂CF₂CF₂H, —CH₂CF₂CF₂H,—CH₂CH₂CF₂CF₃, —CH₂CF₂CFHCF₃, —CF₂CF₂CF₂CF₂H, —CH₂CF₂CF₂CF₂H,—CH₂CH₂CF₂CF₂H, —CH₂CF(CF₃)CF₂H, —CF₂CF₂H, —CH₂CF₂H, and —CF₂CH₃.

Specific examples of the fluorinated ether (I) includeHCF₂CF₂CH₂OCF₂CF₂H, CF₃CF₂CH₂OCF₂CF₂H, HCF₂CF₂CH₂OCF₂CFHCF₃,CF₃CF₂CH₂OCF₂CFHCF₃, C₆F₁₃OCH₃, C₆F₁₃OC₂H₅, C₈F₁₇OCH₃, C₈F₁₇OC₂H₅,CF₃CFHCF₂CH(CH₃) OCF₂CFHCF₃, HCF₂CF₂OCH(C₂H₅)₂, HCF₂CF₂OC₄H₉,HCF₂CF₂OCH₂CH(C₂H₅)₂, and HCF₂CF₂OCH₂CH(CH₃)₂.

In particular, those having HCF₂— or CF₃CFH— at one end or both ends canprovide a fluorinated ether (I) excellent in polarizability and having ahigh boiling point. The boiling point of the fluorinated ether (I) ispreferably 67° C. to 120° C. It is more preferably 80° C. or higher,still more preferably 90° C. or higher.

Such a fluorinated ether (I) may include one or two or more ofCF₃CH₂OCF₂CFHCF₃, CF₃CF₂CH₂OCF₂CFHCF₃, HCF₂CF₂CH₂OCF₂CFHCF₃,HCF₂CF₂CH₂OCH₂CF₂CF₂H, CF₃CFHCF₂CH₂OCF₂CFHCF₃, HCF₂CF₂CH₂OCF₂CF₂H,CF₃CF₂CH₂OCF₂CF₂H, and the like.

In order to advantageously achieve a high boiling point, goodcompatibility with other solvents, and a good solubility of theelectrolyte salt, the fluorinated ether (I) is preferably at least oneselected from the group consisting of HCF₂CF₂CH₂OCF₂CFHCF₃ (boilingpoint: 106° C.), CF₃CF₂CH₂OCF₂CFHCF₃ (boiling point: 82° C.),HCF₂CF₂CH₂OCF₂CF₂H (boiling point: 92° C.), and CF₃CF₂CH₂OCF₂CF₂H(boiling point: 68° C.), more preferably at least one selected from thegroup consisting of HCF₂CF₂CH₂OCF₂CFHCF₃ (boiling point: 106° C.) andHCF₂CF₂CH₂OCF₂CF₂H (boiling point: 92° C.)

Examples of the C3-C6 cyclic ether include 1,3-dioxane,2-methyl-1,3-dioxane, 4-methyl-1,3-dioxane, 1,4-dioxane, and fluorinatedcompounds thereof. Preferred are dimethoxy methane, diethoxy methane,ethoxy methoxy methane, ethylene glycol n-propyl ether, ethylene glycoldi-n-butyl ether, and diethylene glycol dimethyl ether because they havea high ability to solvate with lithium ions and improve the degree ofion dissociation. Particularly preferred are dimethoxy methane, diethoxymethane, and ethoxy methoxy methane because they have a low viscosityand give a high ion conductivity.

Examples of the nitrogen-containing compound include nitrile,fluorine-containing nitrile, carboxylic acid amide, fluorine-containingcarboxylic acid amide, sulfonic acid amide, and fluorine-containingsulfonic acid amide. Also, 1-methyl-2-pyrrolidinone,1-methyl-2-piperidone, 3-methyl-2-oxazilidinone,1,3-dimethyl-2-imidazolidinone, and N-methylsuccinimide may be used.

Examples of the boron-containing compounds include borate esters such astrimethyl borate and triethyl borate, boric acid ethers, and alkylborates.

Examples of the organic silicon-containing compounds include (CH₃)₄—Siand (CH₃)₃—Si—Si(CH₃)₃.

Examples of the fireproof agents (flame retardants) includeorganophosphates and phosphazene-based compounds. Examples of theorganophosphates include fluoroalkyl phosphates, non-fluoroalkylphosphates, and aryl phosphates.

Particularly preferred are fluoroalkyl phosphates because they can showa flame retardant effect even at a small amount.

Specific examples of the fluoroalkyl phosphates include fluorodialkylphosphates disclosed in JP H11-233141 A, alkyl phosphates disclosed inJP H11-283669 A, and fluorotrialkyl phosphates.

Preferred as the fireproof agents (flame retardants) are (CH₃O)₃P═O and(CF₃CH₂O)₃P═O, for example.

The surfactant may be any of cationic surfactants, anionic surfactants,nonionic surfactants, and amphoteric surfactants. In order to achievegood cycle characteristics and rate characteristics, the surfactant ispreferably one containing a fluorine atom.

Preferred examples of such a surfactant containing a fluorine atominclude fluorine-containing carboxylic acid salts represented by thefollowing formula (J):

Rf¹⁵COO⁻M⁺  (J)

(wherein Rf¹⁵ is a C3-C10 fluorine-containing alkyl group which mayoptionally have an ether bond; M⁺ is Li⁺, Na⁺, K⁺, or NHR′₃ ⁺ (where R′smay be the same as or different from each other, and are individually Hor a C1-C3 alkyl group)), and fluorine-containing sulfonic acid saltsrepresented by the following formula (K):

Rf¹⁶SO₃ ⁻M⁺  (K)

(wherein Rf¹⁶ is a C3-C10 fluorine-containing alkyl group which mayoptionally have an ether bond; M⁺ is Li⁺, Na⁺, K⁺, or NHR′₃ ⁺ (where R′smay be the same as or different from each other, and are individually Hor a C1-C3 alkyl group)).

In order to reduce the surface tension of the electrolyte solutionwithout impairing the charge and discharge cycle characteristics, theamount of the surfactant is preferably 0.01 to 2 mass % in theelectrolyte solution.

Examples of the additives for increasing the permittivity includesulfolane, methyl sulfolane, γ-butyrolactone, γ-valerolactone,acetonitrile, and propionitrile.

Examples of the improvers for cycle characteristics and ratecharacteristics include methyl acetate, ethyl acetate, tetrahydrofuran,and 1,4-dioxane.

In order to suppress burst or combustion of batteries in case ofovercharge, for example, the overcharge inhibitor is preferably anovercharge inhibitor having an aromatic ring. Examples of the overchargeinhibitor having an aromatic ring include aromatic compounds such ascyclohexyl benzene, biphenyl, alkyl biphenyl, terphenyl, partiallyhydrogenated terphenyl, t-butyl benzene, t-amyl benzene, diphenyl ether,benzofuran, dibenzofuran, dichloroaniline, and toluene; fluorinatedaromatic compounds such as hexafluorobenzene, fluorobenzene,2-fluorobiphenyl, o-cyclohexyl fluorobenzene, and p-cyclohexylfluorobenzene; and fluoroanisole compounds such as 2,4-difluoroanisole,2,5-difluoroanisole, 2,6-difluoroanisole, and 3,5-difluoroanisole.Preferred are aromatic compounds such as biphenyl, alkyl biphenyl,terphenyl, partially hydrogenated terphenyl, cyclohexyl benzene, t-butylbenzene, t-amyl benzene, diphenyl ether, and dibenzofuran. Thesecompounds may be used alone or in combination of two or more. In thecase of combination use of two or more compounds, preferred is acombination of cyclohexyl benzene and t-butyl benzene or t-amyl benzene,or a combination of at least one oxygen-free aromatic compound selectedfrom biphenyl, alkyl biphenyl, terphenyl, partially hydrogenatedterphenyl, cyclohexyl benzene, t-butyl benzene, t-amyl benzene, and thelike, and at least one oxygen-containing aromatic compound selected fromdiphenyl ether, dibenzofuran, and the like. Such combinations lead togood balance between the overcharge inhibiting characteristics and thehigh-temperature storage characteristics.

In order to suppress burst or combustion of batteries in case ofovercharge, for example, the amount of the overcharge inhibitor ispreferably 0.1 to 5 mass % in the electrolyte solution.

The electrolyte solution of the present invention may further containother known assistants to the extent that the effects of the presentinvention are not impaired. Examples of such known assistants includecarbonate compounds such as erythritan carbonate, spiro-bis-dimethylenecarbonate, and methoxy ethyl-methyl carbonate; carboxylic anhydridessuch as succinic anhydride, glutaric anhydride, maleic anhydride,citraconic anhydride, glutaconic anhydride, itaconic anhydride,diglycolic anhydride, cyclohexanedicarboxylic anhydride,cyclopentanetetracarboxylic dianhydride, and phenylsuccinic anhydride;spiro compounds such as 2,4,8,10-tetraoxaspiro[5.5]undecane and3,9-divinyl-2,4,8,10-tetraoxaspiro[5.5]undecane; sulfur-containingcompounds such as ethylene sulfite, methyl fluorosulfonate, ethylfluorosulfonate, methyl methanesulfonate, ethyl methanesulfonate,busulfan, sulfolene, diphenyl sulfone, N,N-dimethyl methane sulfoneamide, N,N-diethyl methane sulfone amide, and like other chain sulfones,fluorine-containing chain sulfones, chain sulfonates,fluorine-containing chain sulfonates, cyclic sulfones,fluorine-containing cyclic sulfones, halides of sulfonic acid, andhalides of fluorine-containing sulfonic acid; and fluoroaromaticcompounds such as hydrocarbon compounds, including heptane, octane,nonane, decane, and cycloheptane. These compounds may be used alone orin combination of two or more. These assistants can improve the capacityretention characteristics and the cycle characteristics afterhigh-temperature storage.

The electrolyte solution of the present invention may be combined with apolymer material and thereby formed into a gel-like (plasticized), gelelectrolyte solution.

Examples of such a polymer material include conventionally knownpolyethylene oxide and polypropylene oxide, modified products thereof(see JP H08-222270 A, JP 2002-100405 A); polyacrylate-based polymers,polyacrylonitrile, and fluororesins such as polyvinylidene fluoride andvinylidene fluoride-hexafluoropropylene copolymers (see JP H04-506726 T,JP H08-507407 T, JP H10-294131 A); and complexes of any of thesefluororesins and any hydrocarbon resin (see JP H11-35765 A, JP H11-86630A). In particular, polyvinylidene fluoride or a vinylidenefluoride-hexafluoropropylene copolymer is preferably used as a polymermaterial for gel electrolytes.

The electrolyte solution of the present invention may also contain anion conductive compound disclosed in Japanese Patent Application No.2004-301934.

This ion conductive compound is an amorphous fluoropolyether compoundhaving a fluorine-containing group at a side chain and is represented bythe formula (1-1):

A-(D)-B  (1-1)

wherein D is represented by the formula (2-1):

-(D1)_(n)-(FAE)_(m)-(AE)_(p)-(Y)_(q)-  (2-1)

[wherein D1 is an ether unit having a fluoro ether group at a side chainand is represented by the formula (2a):

(wherein Rf is a fluoro ether group which may optionally have across-linkable functional group; and R¹⁰ is a group or a bond that linksRf and the main chain);

FAE is an ether unit having a fluorinated alkyl group at a side chainand is represented by the formula (2b):

(wherein Rfa is a hydrogen atom or a fluorinated alkyl group which mayoptionally have a cross-linkable functional group; and R¹¹ is a group ora bond that links Rfa and the main chain);

AE is an ether unit represented by the formula (2c):

(wherein R¹³ is a hydrogen atom, an alkyl group which may optionallyhave a cross-linkable functional group, an aliphatic cyclic hydrocarbongroup which may optionally have a cross-linkable functional group, or anaromatic hydrocarbon group which may optionally have a cross-linkablefunctional group; and R¹² is a group or a bond that links R¹³ and themain chain);

Y is a unit having at least one selected from the formulas (2d-1) to(2d-3):

n is an integer of 0 to 200;

m is an integer of 0 to 200;

p is an integer of 0 to 10000;

q is an integer of 1 to 100;

n+m is not 0; and

the bonding order of D1, FAE, AE, and Y is not specified]; and

A and B may be the same as or different from each other, and areindividually a hydrogen atom, an alkyl group which may optionally have afluorine atom and/or a cross-linkable functional group, a phenyl groupwhich may optionally have a fluorine atom and/or a cross-linkablefunctional group, a —COOH group, —OR (where R is a hydrogen atom or analkyl group which may optionally have a fluorine atom and/or across-linkable functional group), an ester group, or a carbonate group(if an end of D is an oxygen atom, A and B each are none of a —COOHgroup, —OR, an ester group, and a carbonate group).

The electrolyte solution of the present invention may further containother additives, if necessary. Examples of such other additives includemetal oxides and glass.

The electrolyte solution of the present invention may be prepared by anymethod using the aforementioned components.

The electrolyte solution of the present invention contains at least oneselected from the group consisting of the compounds represented by theformula (1) and the compounds represented by the formula (2). Such aconfiguration enables production of secondary batteries whose internalresistance is less likely to increase even after repeated charge anddischarge and whose cycle capacity retention ratio is high. Accordingly,the electrolyte solution of the present invention can be suitablyapplied to electrochemical devices such as secondary batteries. Such anelectrochemical device or secondary battery including the electrolytesolution of the present invention is also one aspect of the presentinvention.

Examples of the electrochemical devices include lithium ion secondarybatteries, capacitors (electric double-layer capacitors), radicalbatteries, solar cells (in particular, dye-sensitized solar cells), fuelcells, various electrochemical sensors, electrochromic elements,electrochemical switching elements, aluminum electrolytic capacitors,and tantalum electrolytic capacitors. Preferred are lithium ionsecondary batteries and electric double-layer capacitors.

In the following, a lithium ion secondary battery is described as anexample of the electrochemical device or secondary battery of thepresent invention.

The lithium ion secondary battery includes a positive electrode, anegative electrode, and the aforementioned electrolyte solution.

<Positive Electrode>

The positive electrode is formed from a positive electrode mixturecontaining a positive electrode active material, which is a material ofthe positive electrode, and a current collector.

The positive electrode active material may be any material that canelectrochemically occlude and release lithium ions. For example, asubstance containing lithium and at least one transition metal ispreferred. Specific examples thereof include lithium-containingtransition metal complex oxides and lithium-containing transition metalphosphoric acid compounds. In particular, the positive electrode activematerial is preferably a lithium-containing transition metal complexoxide that generates a high voltage.

Examples of the lithium-containing transition metal complex oxidesinclude

lithium-manganese spinel complex oxides represented by the formula (L):Li_(a)Mn_(2−b)M¹ _(b)O₄ (wherein 0.9≦a; 0≦b≦1.5; and M¹ is at least onemetal selected from the group consisting of Fe, Co, Ni, Cu, Zn, Al, Sn,Cr, V, Ti, Mg, Ca, Sr, B, Ga, In, Si, and Ge),

lithium-nickel complex oxides represented by the formula (M):LiNi_(1−c)M² _(c)O₂ (wherein 0≦c≦0.5; and M² is at least one metalselected from the group consisting of Fe, Co, Mn, Cu, Zn, Al, Sn, Cr, V,Ti, Mg, Ca, Sr, B, Ga, In, Si, and Ge), and

lithium-cobalt complex oxides represented by the formula (N):LiCo_(1−d)M³ _(d)O₂ (wherein 0≦d≦0.5; and M³ is at least one metalselected from the group consisting of Fe, Ni, Mn, Cu, Zn, Al, Sn, Cr, V,Ti, Mg, Ca, Sr, B, Ga, In, Si, and Ge).

In order to provide a high-power lithium ion secondary battery having ahigh energy density, preferred is LiCoO₂, LiMnO₂, LiNiO₂, LiMn₂O₄,LiNi_(0.8)CO_(0.15)Al_(0.5)O₂, LiNi_(0.5)Mn_(1.5)O₄, orLiNi_(1/3)Co_(1/3)Mn_(1/3)O₂.

Examples of other positive electrode active materials include LiFePO₄,LiNi_(0.8)CO_(0.2)O₂, Li_(1.2)Fe_(0.4)Mn_(0.4)O₂, LiNi_(0.5)Mn_(0.5)O₂,and LiV₃O₆.

In order to improve the continuous charge characteristics, the positiveelectrode active material preferably contains lithium phosphate. The useof lithium phosphate may be achieved in any manner, and the positiveelectrode active material and lithium phosphate are preferably used inadmixture. The lower limit of the amount of lithium phosphate in the sumof the amounts of the positive electrode active material and the lithiumphosphate is preferably 0.1 mass % or more, more preferably 0.3 mass %or more, still more preferably 0.5 mass % or more, whereas the upperlimit thereof is preferably 10 mass % or less, more preferably 8 mass %or less, still more preferably 5 mass % or less.

To the surface of the positive electrode active material may be attacheda substance having a composition different from the positive electrodeactive material.

Examples of the substance attached to the surface include oxides such asaluminum oxide, silicon oxide, titanium oxide, zirconium oxide,magnesium oxide, calcium oxide, boron oxide, antimony oxide, and bismuthoxide; sulfates such as lithium sulfate, sodium sulfate, potassiumsulfate, magnesium sulfate, calcium sulfate, and aluminum sulfate;carbonates such as lithium carbonate, calcium carbonate, magnesiumcarbonate; and carbon.

Such a substance may be attached to the surface of the positiveelectrode active material by, for example, a method of dissolving orsuspending the substance in a solvent, impregnating the solution orsuspension into the positive electrode active material, and drying theimpregnated material; a method of dissolving or suspending a precursorof the substance in a solvent, impregnating the solution or suspensioninto the positive electrode active material, and reacting the materialand the precursor by heating; or a method of adding the substance to aprecursor of the positive electrode active material and simultaneouslysintering the materials. In the case where carbon is to be attached, acarbonaceous material in the form of activated carbon, for example, maybe mechanically attached to the surface afterward.

The lower limit of the amount (in terms of mass) of the substanceattached to the surface relative to the amount of the positive electrodeactive material is preferably 0.1 ppm or more, more preferably 1 ppm ormore, still more preferably 10 ppm or more, whereas the upper limitthereof is preferably 20% or less, more preferably 10% or less, stillmore preferably 5% or less. The substance attached to the surfacesuppresses the oxidation of the electrolyte solution on the surface ofthe positive electrode active material, so that the battery life isimproved. If the amount thereof is too small, the substance fails tosufficiently provide the effect. If the amount thereof is too large, thesubstance may hinder the entrance and exit of lithium ions, so that theresistance may be increased.

Particles of the positive electrode active material may have anyconventionally used shape, such as an agglomerative shape, a polyhedralshape, a spherical shape, an ellipsoidal shape, a plate shape, a needleshape, or a pillar shape. The primary particles may agglomerate to formsecondary particles.

The positive electrode active material has a tap density of preferably0.5 g/cm³ or higher, more preferably 0.8 g/cm³ or higher, still morepreferably 1.0 g/cm³ or higher. If the tap density of the positiveelectrode active material is below the lower limit, an increased amountof a dispersion medium is required, as well as increased amounts of aconductive material and a binder are required in formation of a positiveelectrode active material layer. Thus, the filling rate of the positiveelectrode active material into the positive electrode active materiallayer may be limited and the battery capacity may be limited. With acomplex oxide powder having a high tap density, a positive electrodeactive material layer with a high density can be formed. The tap densityis preferably as high as possible and has no upper limit, in general.Still, if the tap density is too high, diffusion of lithium ions in thepositive electrode active material layer with the electrolyte solutionserving as a diffusion medium may function as a rate-determining step,so that the load characteristics may be easily impaired. Thus, the upperlimit of the tap density is preferably 4.0 g/cm³ or lower, morepreferably 3.7 g/cm³ or lower, still more preferably 3.5 g/cm³ or lower.

The tap density is determined as a powder filling density (tap density)g/cc when 5 to 10 g of the positive electrode active material powder isfilled into a 10-ml glass graduated cylinder and the cylinder is tapped200 times with a stroke of about 20 mm.

The particles of the positive electrode active material have a mediansize d50 (if the primary particles agglomerate to form secondaryparticles, the secondary particle size) of preferably 0.3 μm or greater,more preferably 0.5 μm or greater, still more preferably 0.8 μm orgreater, most preferably 1.0 μm or greater, whereas the median size d50is preferably 30 μm or smaller, more preferably 27 μm or smaller, stillmore preferably 25 μm or smaller, most preferably 22 μm or smaller. Ifthe median size is below the lower limit, products with a high tapdensity may not be obtained. If the median size exceeds the upper limit,diffusion of lithium in the positive electrode active material layer maytake a long time, so that the battery performance may be poor or streaksmay be formed in formation of positive electrodes for batteries, i.e.,when the active material and components such as a conductive materialand a binder are formed into slurry by adding a solvent and the slurryis applied in the form of a film, for example. Mixing two or morepositive electrode active materials having different median sizes d50leads to further improved filling in formation of positive electrodes.

The median size d50 is determined using a known laserdiffraction/scattering particle size distribution analyzer. In the caseof using LA-920 (Horiba, Ltd.) as the particle size distributionanalyzer, the dispersion medium used in the measurement is a 0.1 mass %sodium hexametaphosphate aqueous solution and the measurement refractiveindex is set to 1.24 after 5-minute ultrasonic dispersion.

If the primary particles agglomerate to form secondary particles, theaverage primary particle size of the positive electrode active materialis preferably 0.05 μm or greater, more preferably 0.1 μm or greater,still more preferably 0.2 μm or greater. The upper limit thereof ispreferably 5 μm or smaller, more preferably 4 μm or smaller, still morepreferably 3 μm or smaller, most preferably 2 μm or smaller. If theaverage primary particle size exceeds the upper limit, sphericalsecondary particles are difficult to form, which may have a badinfluence on the powder filling or may cause a great reduction in thespecific surface area. Thus, the battery performance such as outputcharacteristics is more likely to be impaired. In contrast, if theaverage primary particle size is below the lower limit, the crystalsusually do not sufficiently grow. Thus, charge and discharge may bepoorly reversible.

The primary particle size is measured by scanning electron microscopic(SEM) observation. Specifically, the primary particle size is determinedas follows. A photograph at a magnification of 10000× is first taken.Any 50 primary particles are selected and the maximum length between theleft and right boundary lines of each primary particle is measured alongthe horizontal line. Then, the average value of the maximum lengths iscalculated, which is defined as the primary particle size.

The positive electrode active material has a BET specific surface areaof preferably 0.1 m²/g or larger, more preferably 0.2 m²/g or larger,still more preferably 0.3 m²/g or larger. The BET specific surface areais preferably 50 m²/g or smaller, more preferably 40 m²/g or smaller,still more preferably 30 m²/g or smaller. If the BET specific surfacearea is smaller than the above range, the battery performance may beeasily impaired. If it is larger than the above range, the tap densityis less likely to be high and formation of the positive electrode activematerial layer may involve a difficulty in applying the material.

The BET specific surface area is defined by a value determined by singlepoint BET nitrogen adsorption utilizing a gas flow method using asurface area analyzer (e.g., fully automatic surface area measurementdevice, Ohkura Riken Co., Ltd.), a sample pre-dried in the stream ofnitrogen at 150° C. for 30 minutes, and a nitrogen-helium gas mixturewith the nitrogen pressure relative to the atmospheric pressure beingaccurately adjusted to 0.3.

When the lithium ion secondary battery is used as a large-size lithiumion secondary battery for hybrid vehicles or distributed generation, itis required to achieve a high output. Thus, the particles of thepositive electrode active material preferably mainly include secondaryparticles.

The particles of the positive electrode active material preferablyinclude 0.5 to 7.0 vol % of fine particles having an average secondaryparticle size of 40 μm or smaller and having an average primary particlesize of 1 μm or smaller. Containing fine particles having an averageprimary particle size of 1 μm or smaller leads to an increase in thecontact area with the electrolyte solution and more rapid diffusion oflithium ions between the electrode and the electrolyte solution. As aresult, the output performance of the battery can be improved.

The positive electrode active material can be produced by any usualmethod of producing inorganic compounds. In particular, a spherical orellipsoidal active material can be produced by various methods. Forexample, a material substance of transition metal is dissolved orcrushed and dispersed in a solvent such as water, and the pH of thesolution or dispersion is adjusted under stirring to form a sphericalprecursor. The precursor is recovered and, if necessary, dried. Then, aLi source such as LiOH, Li₂CO₃, or LiNO₃ is added thereto and themixture is sintered at high temperature, thereby providing an activematerial.

In order to produce a positive electrode, the aforementioned positiveelectrode active materials may be used alone or in any combination withone or more having different compositions at any ratio. Preferredexamples of the combination in this case include a combination withLiCoO₂ and LiMn₂O₄ in which part of Mn may optionally be replaced by adifferent transition metal (e.g., LiNi_(0.33)Co_(0.33)Mn_(0.33)O₂), anda combination with LiCoO₂ in which part of Co may optionally be replacedby a different transition metal.

In order to achieve a high battery capacity, the amount of the positiveelectrode active material is preferably 50 to 99 mass %, more preferably80 to 99 mass %, of the positive electrode mixture. The amount of thepositive electrode active material in the positive electrode activematerial layer is preferably 80 mass % or more, more preferably 82 mass% or more, particularly preferably 84 mass % or more. The amount thereofis preferably 99 mass % or less, more preferably 98 mass % or less. Toosmall an amount of the positive electrode active material in thepositive electrode active material layer may lead to an insufficientelectric capacity. In contrast, too large an amount thereof may lead toan insufficient strength of the resulting positive electrode.

The positive electrode mixture preferably further includes a binder, athickening agent, and a conductive material.

The binder may be any material that is safe against a solvent to be usedin production of electrodes and the electrolyte solution. Examplesthereof include polyvinylidene fluoride, polytetrafluoroethylene,polyethylene, polypropylene, SBR (styrene-butadiene rubber), isoprenerubber, butadiene rubber, ethylene-acrylic acid copolymers,ethylene-methacrylic acid copolymers, polyethylene terephthalate,polymethyl methacrylate, polyimide, aromatic polyamide, cellulose, nitrocellulose, NBR (acrylonitrile-butadiene rubber), fluororubber,ethylene-propylene rubber, styrene-butadiene-styrene block copolymers orhydrogenated products thereof, EPDM (ethylene-propylene-dieneterpolymers), styrene-ethylene-butadiene-ethylene copolymers,styrene-isoprene-styrene block copolymers or hydrogenated productsthereof, syndiotactic-1,2-polybutadiene, polyvinyl acetate,ethylene-vinyl acetate copolymers, propylene-α-olefin copolymers,fluorinated polyvinylidene fluoride, polytetrafluoroethylene-ethylenecopolymers, and polymer compositions having an ion conductivity ofalkali metal ions (especially, lithium ions). These agents may be usedalone or in any combination of two or more at any ratio.

The amount of the binder, which is expressed as the proportion of thebinder in the positive electrode active material layer, is usually 0.1mass % or more, preferably 1 mass % or more, more preferably 1.5 mass %or more. The proportion is also usually 80 mass % or less, preferably 60mass % or less, still more preferably 40 mass % or less, most preferably10 mass % or less. Too low a proportion of the binder may fail tosufficiently hold the positive electrode active material so that theresulting positive electrode may have an insufficient mechanicalstrength, resulting in deteriorated battery performance such as cyclecharacteristics. In contrast, too high a proportion thereof may lead toa reduction in battery capacity and conductivity.

Examples of the thickening agent include carboxymethyl cellulose, methylcellulose, hydroxymethyl cellulose, ethyl cellulose, polyvinyl alcohol,oxidized starch, monostarch phosphate, casein, and salts thereof. Theseagents may be used alone or in any combination of two or more at anyratio.

The proportion of the thickening agent relative to the active materialis usually 0.1 mass % or more, preferably 0.2 mass % or more, morepreferably 0.3 mass % or more. It is usually 5 mass % or less,preferably 3 mass % or less, more preferably 2 mass % or less. If theproportion thereof is below this range, easiness of application may besignificantly impaired. If the proportion is above this range, theproportion of the active material in the positive electrode activematerial layer decreases, so that the capacity of the battery maydecrease or the resistance between the positive electrode activematerials may increase.

The conductive material may be any known conductive material. Specificexamples thereof include metal materials such as copper and nickel, andcarbon materials such as graphite (e.g., natural graphite, artificialgraphite), carbon black (e.g., acetylene black), and amorphous carbon(e.g., needle coke). These materials may be used alone or in anycombination of two or more at any ratio. The conductive material is usedsuch that the amount thereof in the positive electrode active materiallayer is usually 0.01 mass % or more, preferably 0.1 mass % or more,more preferably 1 mass % or more, whereas the amount thereof is usually50 mass % or less, preferably 30 mass % or less, more preferably 15 mass% or less. If the amount thereof is below this range, the conductivitymay be insufficient. In contrast, if the amount thereof is above thisrange, the battery capacity may decrease.

The solvent for forming slurry may be any solvent that can dissolve ordisperse therein the positive electrode active material, the conductivematerial, and the binder, as well as a thickening agent used ifnecessary. The solvent may be either of an aqueous solvent or an organicsolvent. Examples of the aqueous medium include water and solventmixtures of an alcohol and water. Examples of the organic medium includealiphatic hydrocarbons such as hexane; aromatic hydrocarbons such asbenzene, toluene, xylene, and methyl naphthalene; heterocyclic compoundssuch as quinoline and pyridine; ketones such as acetone, methyl ethylketone, and cyclohexanone; esters such as methyl acetate and methylacrylate; amines such as diethylene triamine andN,N-dimethylaminopropylamine; ethers such as diethyl ether, propyleneoxide, and tetrahydrofuran (THF); amides such as N-methylpyrrolidone(NMP), dimethyl formamide, and dimethyl acetamide; and aprotic polarsolvents such as hexamethyl phospharamide and dimethyl sulfoxide.

The current collector for positive electrodes may be formed from anymetal material such as aluminum, titanium, tantalum, stainless steel, ornickel, or any alloy thereof; or any carbon material such as carboncloth or carbon paper. Preferred is any metal material, especiallyaluminum or alloy thereof.

In the case of a metal material, the current collector may be in theform of metal foil, metal cylinder, metal coil, metal plate, metal film,expanded metal, punched metal, metal foam, or the like. In the case of acarbon material, it may be in the form of carbon plate, carbon film,carbon cylinder, or the like. Preferred among these is a metal film. Thefilm may be in the form of mesh, as appropriate. The film may have anythickness, and the thickness is usually 1 μm or greater, preferably 3 μmor greater, more preferably 5 μm or greater, whereas the thickness isusually 1 mm or smaller, preferably 100 μm or smaller, more preferably50 μm or smaller. If the film is thinner than this range, it may have aninsufficient strength as a current collector. In contrast, if the filmis thicker than this range, it may have poor handleability.

In order to reduce the electronic contact resistance between the currentcollector and the positive electrode active material layer, the currentcollector also preferably has a conductive auxiliary agent applied onthe surface thereof. Examples of the conductive auxiliary agent includecarbon and noble metals such as gold, platinum, and silver.

The ratio between the thicknesses of the current collector and thepositive electrode active material layer may be any value, and the ratio{(thickness of positive electrode active material layer on one sideimmediately before injection of electrolyte solution)/(thickness ofcurrent collector)} is preferably 20 or lower, more preferably 15 orlower, most preferably 10 or lower. The ratio is also preferably 0.5 orhigher, more preferably 0.8 or higher, most preferably 1 or higher. Ifthe ratio exceeds this range, the current collector may generate heatdue to Joule heating during high-current-density charge and discharge.If the ratio is below this range, the ratio by volume of the currentcollector to the positive electrode active material is so high that thecapacity of the battery may decrease.

The positive electrode may be produced by a usual method. One example ofthe production method is a method in which the positive electrode activematerial is mixed with the aforementioned binder, thickening agent,conductive material, solvent, and other components to form a slurry-likepositive electrode mixture, and then this mixture is applied to acurrent collector, dried, and pressed so as to be densified.

The densification may be achieved using a manual press or a roll press,for example. The density of the positive electrode active material layeris preferably 1.5 g/cm³ or higher, more preferably 2 g/cm³ or higher,still more preferably 2.2 g/cm³ or higher, whereas the density thereofis preferably 5 g/cm³ or lower, more preferably 4.5 g/cm³ or lower,still more preferably 4 g/cm³ or lower. If the density is above thisrange, the permeability of the electrolyte solution toward the vicinityof the interface between the current collector and the active materialmay decrease, in particular, the charge and discharge characteristics athigh current density may be impaired, so that a high output may not beachieved. If the density is below this range, the conductivity betweenthe active materials may decrease and the resistance of the battery mayincrease, so that a high output may not be achieved.

In order to improve the stability at high output and high temperature,the area of the positive electrode active material layer is preferablylarge relative to the outer surface area of an external case of thebattery in the case of using the electrolyte solution of the presentinvention. Specifically, the sum of the areas of the positive electrodesis preferably 15 times or more, more preferably 40 times or more,greater than the surface area of the external case of the secondarybattery. For closed, square-shaped cases, the outer surface area of anexternal case of the battery herein refers to the total area calculatedfrom the dimension of length, width, and thickness of the case portioninto which a power-generating element is filled except for a protrudingportion of a terminal. For closed, cylinder-like cases, the outersurface area of an external case of the battery herein refers to ageometric surface area of an approximated cylinder of the case portioninto which a power-generating element is filled except for a protrudingportion of a terminal. The sum of the areas of the positive electrodesherein refers to a geometric surface area of a positive electrodemixture layer opposite to a mixture layer including the negativeelectrode active material. For structures including a current collectorfoil and positive electrode mixture layers on both sides of the currentcollector, the sum of the areas of the positive electrodes is the sum ofthe areas calculated on the respective sides.

The positive electrode plate may have any thickness. In order to achievea high capacity and a high output, the lower limit of the thickness ofthe mixture layer on one side of the current collector excluding thethickness of the base metal foil is preferably 10 μm or greater, morepreferably 20 μm or greater, whereas the thickness thereof is preferably500 μm or smaller, more preferably 450 μm or smaller.

To the surface of the positive electrode plate may be attached asubstance having a different composition. Examples of the substanceattached to the surface include oxides such as aluminum oxide, siliconoxide, titanium oxide, zirconium oxide, magnesium oxide, calcium oxide,boron oxide, antimony oxide, and bismuth oxide; sulfates such as lithiumsulfate, sodium sulfate, potassium sulfate, magnesium sulfate, calciumsulfate, and aluminum sulfate; carbonates such as lithium carbonate,calcium carbonate, and magnesium carbonate; and carbon.

<Negative Electrode>

The negative electrode includes a negative electrode mixture including anegative electrode active material, and a current collector.

Examples of the negative electrode active material include carbonaceousmaterials that can occlude and release lithium such as pyrolysates oforganic matter under various pyrolysis conditions, artificial graphite,and natural graphite; metal oxide materials that can occlude and releaselithium such as tin oxide and silicon oxide; lithium metals; variouslithium alloys; and lithium-containing metal complex oxide materials.Two or more of these negative electrode active materials may be used inadmixture.

The carbonaceous material that can occlude and release lithium ispreferably artificial graphite produced by high-temperature treatment ofeasily graphitizable pitch from various materials, purified naturalgraphite, or a material obtained by surface-treating such graphite withpitch or other organic matter and then carbonizing the surface-treatedgraphite. In order to achieve a good balance between the initialirreversible capacity and the high-current-density charge and dischargecharacteristics, it is more preferably selected from carbonaceousmaterials obtained by one or more heat treatments at 400° C. to 3200° C.on natural graphite, artificial graphite, artificial carbonaceoussubstances, or artificial graphite substances; carbonaceous materialswhich allow the negative electrode active material layer to include atleast two or more carbonaceous matters having different crystallinitiesand/or has an interface between the carbonaceous matters having thedifferent crystallinities; and carbonaceous materials which allow thenegative electrode active material layer to have an interface between atleast two or more carbonaceous matters having different orientations.These carbonaceous materials may be used alone or in any combination oftwo or more at any ratio.

Examples of the carbonaceous materials obtained by one or more heattreatments at 400° C. to 3200° C. on artificial carbonaceous substancesor artificial graphite substances include natural graphite, coal-basedcoke, petroleum-based coke, coal-based pitch, petroleum-based pitch, andthose prepared by oxidizing these pitches; needle coke, pitch coke, andcarbon materials prepared by partially graphitizing these cokes;pyrolysates of organic matter such as furnace black, acetylene black,and pitch-based carbon fibers; carbonizable organic matter and carbidesthereof; and solutions prepared by dissolving carbonizable organicmatter in a low-molecular-weight organic solvent such as benzene,toluene, xylene, quinoline, or n-hexane, and carbides thereof.

The metal material (excluding lithium-titanium complex oxides) to beused as the negative electrode active material may be any compound thatcan occlude and release lithium, and examples thereof include simplelithium, simple metals and alloys that constitute lithium alloys, andoxides, carbides, nitrides, silicides, sulfides, and phosphides thereof.The simple metals and alloys constituting lithium alloys are preferablymaterials containing any of metal and semi-metal elements in the Groups13 and 14, more preferably simple metal of aluminum, silicon, and tin(hereinafter, referred to as “specific metal elements”), and alloys andcompounds containing any of these atoms. These materials may be usedalone or in combination of two or more at any ratio.

Examples of the negative electrode active material having at least oneatom selected from the specific metal elements include simple metal ofany one specific metal element, alloys of two or more specific metalelements, alloys of one or two or more specific metal elements and oneor two or more other metal elements, compounds containing one or two ormore specific metal elements, and composite compounds such as oxides,carbides, nitrides, silicides, sulfides, and phosphides of thecompounds. Use of such a simple metal, alloy, or metal compound as thenegative electrode active material can give a high capacity tobatteries.

Examples thereof further include compounds in which any of the abovecomposite compounds are complexly bonded with several elements such assimple metals, alloys, and nonmetal elements. Specifically, in the caseof silicon or tin, for example, an alloy of this element and a metalthat does not serve as a negative electrode can be used. In the case oftin, for example, a composite compound including a combination of 5 or 6elements, including tin, a metal (excluding silicon) that serves as anegative electrode, a metal that does not serve as a negative electrode,and a nonmetal element, can be used.

Specific examples thereof include simple Si, SiB₄, SiB₆, Mg₂Si, Ni₂Si,TiSi₂, MoSi₂, CoSi₂, NiSi₂, CaSi₂, CrSi₂, Cu₆Si, FeSi₂, MnSi₂, NbSi₂,TaSi₂, VSi₂, WSi₂, ZnSi₂, SiC, Si₃N₄, Si₂N₂O, SiOv (0<v≦2), LiSiO,simple tin, SnSiO₃, LiSnO, Mg₂Sn, and SnOw (0<w≦2).

Examples thereof further include composite materials of Si or Sn used asa first constitutional element, and second and third constitutionalelements. The second constitutional element is at least one selectedfrom cobalt, iron, magnesium, titanium, vanadium, chromium, manganese,nickel, copper, zinc, gallium, and zirconium, for example. The thirdconstitutional element is at least one selected from boron, carbon,aluminum, and phosphorus, for example.

In order to achieve a high battery capacity and excellent batterycharacteristics, the metal material is preferably simple silicon or tin(which may contain trace impurities), SiOv (0<v≦2), SnOw (0≦w≦2), aSi—Co—C composite material, a Si—Ni—C composite material, a Sn—Co—Ccomposite material, or a Sn—Ni—C composite material.

The lithium-containing metal complex oxide material to be used as thenegative electrode active material may be any material that can occludeand release lithium. In order to achieve good high-current-densitycharge and discharge characteristics, materials containing titanium andlithium are preferred, lithium-containing metal complex oxide materialscontaining titanium are more preferred, and complex oxides of lithiumand titanium (hereinafter, abbreviated as “lithium titanium complexoxides”) are still more preferred. In other words, use of aspinel-structured lithium titanium complex oxide contained in thenegative electrode active material for electrolyte batteries isparticularly preferred because such a compound markedly reduces theoutput resistance.

Preferred examples of the lithium titanium complex oxides includecompounds represented by the formula (O):

Li_(x)Ti_(y)M_(z)O₄  (O)

wherein M is at least one element selected from the group consisting ofNa, K, Co, Al, Fe, Ti, Mg, Cr, Ga, Cu, Zn, and Nb.

Particularly preferred compositions represented by the formula (O) arethose satisfying one of the following:

(i) 1.2≦x≦1.4, 1.5≦y≦1.7, z=0(ii) 0.9≦x≦1.1, 1.9≦y≦2.1, z=0(iii) 0.7≦x≦0.9, 2.1≦y≦2.3, z=0because the compound structure satisfying any of these compositionsgives good balance of the battery performance.

Particularly preferred representative composition of the compound isLi_(4/3)Ti_(5/3)O₄ corresponding to the composition (i), Li₁Ti₂O₄corresponding to the composition (ii), and Li_(4/5)Ti_(11/5)O₄corresponding to the composition (iii).

Preferred examples of the structure satisfying Z≠0 includeLi_(4/3)Ti_(4/3)Al_(1/3)O₄.

The negative electrode mixture preferably further contains a binder, athickening agent, and a conductive material.

Examples of the binder include the same binders as those mentioned forthe positive electrode. The proportion of the binder relative to thenegative electrode active material is preferably 0.1 mass % or more,more preferably 0.5 mass % or more, particularly preferably 0.6 mass %or more. The proportion is also preferably 20 mass % or less, morepreferably 15 mass % or less, still more preferably 10 mass % or less,particularly preferably 8 mass % or less. If the proportion of thebinder relative to the negative electrode active material exceeds theabove range, a large amount of the binder may fail to contribute to thebattery capacity, so that the battery capacity may decrease. If theproportion is lower than the above range, the negative electrode mayhave a lowered strength.

In particular, in the case of using a rubbery polymer typified by SBR asa main component, the proportion of the binder relative to the negativeelectrode active material is usually 0.1 mass % or more, preferably 0.5mass % or more, more preferably 0.6 mass % or more, whereas theproportion thereof is usually 5 mass % or less, preferably 3 mass % orless, more preferably 2 mass % or less. In the case of using afluoropolymer typified by polyvinylidene fluoride as a main component,the proportion of the binder relative to the negative electrode activematerial is usually 1 mass % or more, preferably 2 mass % or more, morepreferably 3 mass % or more, whereas the proportion thereof is usually15 mass % or less, preferably 10 mass % or less, more preferably 8 mass% or less.

Examples of the thickening agent include the same thickening agents asthose mentioned for the positive electrode. The proportion of thethickening agent relative to the negative electrode active material isusually 0.1 mass % or more, preferably 0.5 mass % or more, still morepreferably 0.6 mass % or more, whereas the proportion thereof is usually5 mass % or less, preferably 3 mass % or less, still more preferably 2mass % or less. If the proportion of the thickening agent relative tothe negative electrode active material is below the range, easiness ofapplication may be significantly impaired. If the proportion thereof isabove the range, the proportion of the negative electrode activematerial in the negative electrode active material layer decreases, sothat the capacity of the battery may decrease or the resistance betweenthe negative electrode active materials may increase.

Examples of the conductive material of the negative electrode includemetal materials such as copper and nickel; and carbon materials such asgraphite and carbon black.

The solvent for forming slurry may be any solvent that can dissolve ordisperse the negative electrode active material and the binder, and athickening agent and a conductive material that are used as necessary.The slurry-forming solvent may be an aqueous solvent or an organicsolvent.

Examples of the aqueous solvent include water and alcohols. Examples ofthe organic solvent include N-methylpyrrolidone (NMP), dimethylformamide, dimethyl acetamide, methyl ethyl ketone, cyclohexanone,methyl acetate, methyl acrylate, diethyl triamine, N,N-dimethylaminopropyl amine, tetrahydrofuran (THF), toluene, acetone, diethylether, dimethyl acetamide, hexamethyl phospharamide, dimethyl sulfoxide,benzene, xylene, quinoline, pyridine, methyl naphthalene, and hexane.

Examples of the material of the current collector for negativeelectrodes include copper, nickel, and stainless steel. For easyprocessing of the material into a film and low cost, copper ispreferred.

The current collector usually has a thickness of 1 μm or larger,preferably 5 μm or larger. The thickness is also usually 100 μm orsmaller, preferably 50 μm or smaller. Too thick a negative electrodecurrent collector may cause an excessive reduction in capacity of thewhole battery, whereas too thin a current collector may be difficult tohandle.

The negative electrode may be produced by a usual method. One example ofthe production method is a method in which the negative electrodematerial is mixed with the aforementioned binder, thickening agent,conductive material, solvent, and other components to form a slurry-likemixture, and then this mixture is applied to a current collector, dried,and pressed so as to be densified. In the case of using an alloyedmaterial, a thin film layer containing the above negative electrodeactive material (negative electrode active material layer) can beproduced by vapor deposition, sputtering, plating, or the liketechnique.

The electrode formed from the negative electrode active material mayhave any structure. The density of the negative electrode activematerial existing on the current collector is preferably 1 g·cm⁻³ orhigher, more preferably 1.2 g·cm⁻³ or higher, particularly preferably1.3 g·cm⁻³ or higher, whereas the density thereof is preferably 2.2g·cm⁻³ or lower, more preferably 2.1 g·cm⁻³ or lower, still morepreferably 2.0 g·cm⁻³ or lower, particularly preferably 1.9 g·cm⁻³ orlower. If the density of the negative electrode active material existingon the current collector exceeds the above range, the particles of thenegative electrode active material may be broken, the initialirreversible capacity may increase, and the permeability of theelectrolyte solution toward the vicinity of the interface between thecurrent collector and the negative electrode active material may beimpaired, so that the high-current-density charge and dischargecharacteristics may be impaired. If the density thereof is below theabove range, the conductivity between the negative electrode activematerials may be impaired, the resistance of the battery may increase,and the capacity per unit volume may decrease.

The thickness of the negative electrode plate is a design matter inaccordance with the positive electrode plate to be used, and may be anyvalue. The thickness of the mixture layer excluding the thickness of thebase metal foil is usually 15 μm or greater, preferably 20 μm orgreater, more preferably 30 μm or greater, whereas the thickness thereofis usually 300 μm or smaller, preferably 280 μm or smaller, morepreferably 250 μm or smaller.

To the surface of the negative electrode plate may be attached asubstance having a different composition. Examples of the substanceattached to the surface include oxides such as aluminum oxide, siliconoxide, titanium oxide, zirconium oxide, magnesium oxide, calcium oxide,boron oxide, antimony oxide, and bismuth oxide; sulfates such as lithiumsulfate, sodium sulfate, potassium sulfate, magnesium sulfate, calciumsulfate, and aluminum sulfate; and carbonates such as lithium carbonate,calcium carbonate, and magnesium carbonate.

<Separator>

The lithium ion secondary battery preferably further includes aseparator.

The separator may be formed from any known material and may have anyknown shape as long as the resulting separator is stable to theelectrolyte solution and is excellent in a liquid-retaining ability. Theseparator is preferably in the form of a porous sheet or a nonwovenfabric which is formed from a material stable to the electrolytesolution of the present invention, such as resin, glass fiber, orinorganic matter, and which is excellent in a liquid-retaining ability.

Examples of the material of a resin or glass-fiber separator includepolyolefins such as polyethylene and polypropylene, aromatic polyamide,polytetrafluoroethylene, polyether sulfone, and glass filters. Thesematerials may be used alone or in any combination of two or more at anyratio, for example, in the form of a polypropylene/polyethylene bilayerfilm or a polypropylene/polyethylene/polypropylene trilayer film. Inorder to achieve good permeability of the electrolyte solution and agood shut-down effect, the separator is particularly preferably a poroussheet or a nonwoven fabric formed from polyolefin such as polyethyleneor polypropylene.

The separator may have any thickness, and the thickness is usually 1 μmor larger, preferably 5 μm or larger, more preferably 8 μm or larger,whereas the thickness is usually 50 μm or smaller, preferably 40 μm orsmaller, more preferably 30 μm or smaller. If the separator is thinnerthan the above range, the insulation and mechanical strength may bepoor. If the separator is thicker than the above range, not only thebattery performance, such as rate characteristics, may be poor but alsothe energy density of the whole electrolyte battery may be low.

If the separator is a porous one such as a porous sheet or a nonwovenfabric, the separator may have any porosity. The porosity is usually 20%or higher, preferably 35% or higher, more preferably 45% or higher,whereas the porosity is usually 90% or lower, preferably 85% or lower,more preferably 75% or lower. If the porosity is lower than the range,the film resistance tends to be high and the rate characteristics tendto be poor. If the porosity is higher than the range, the mechanicalstrength of the separator tends to be low and the insulation tends to bepoor.

The separator may also have any average pore size. The average pore sizeis usually 0.5 μm or smaller, preferably 0.2 μm or smaller, whereas theaverage pore size is usually 0.05 μm or larger. If the average pore sizeexceeds the range, short circuits may easily occur. If the average poresize is lower than the range, the film resistance may be high and therate characteristics may be poor.

Examples of the inorganic material include oxides such as alumina andsilicon dioxide, nitrides such as aluminum nitride and silicon nitride,and sulfates such as barium sulfate and calcium sulfate. The inorganicmaterial is in the form of particles or fibers.

The separator is in the form of a thin film such as a nonwoven fabric, awoven fabric, or a microporous film. The thin film favorably has a poresize of 0.01 to 1 μm and a thickness of 5 to 50 μm. In addition to theform of the above separate thin film, the separator may have a structurein which a composite porous layer containing particles of the aboveinorganic material is formed on the surface of one or both of thepositive and negative electrodes using a resin binder. For example,alumina particles having a 90% particle size of smaller than 1 μm areapplied to the respective surfaces of the positive electrode withfluororesin used as a binder to form a porous layer.

<Battery Design>

The electrode group may be either a laminated structure including theabove positive and negative electrode plates with the above separator inbetween, or a wound structure including the above positive and negativeelectrode plates in spiral with the above separator in between. Theproportion of the volume of the electrode group in the battery internalvolume (hereinafter, referred to as an electrode group proportion) isusually 40% or higher, preferably 50% or higher, whereas the proportionthereof is usually 90% or lower, preferably 80% or lower.

If the electrode group proportion is lower than the above range, thebattery capacity may be low. If the electrode group proportion exceedsthe above range, the battery may have small space for voids. Thus, whenthe battery temperature rises to high temperature, the components mayswell or the liquid fraction of the electrolyte solution shows a highvapor pressure, so that the internal pressure rises. As a result, thebattery characteristics such as charge and discharge repeatability andthe high-temperature storageability may be impaired, and a gas-releasingvalve for releasing the internal pressure toward the outside may work.

The current collecting structure may be any structure. In order to moreeffectively improve the high-current-density charge and dischargecharacteristics by the electrolyte solution of the present invention,the current collecting structure is preferably a structure which reducesthe resistances at wiring portions and jointing portions. When theinternal resistance is reduced in such a manner, the effects of usingthe electrolyte solution of the present invention can particularlyfavorably be achieved.

In an electrode group having the layered structure, the metal coreportions of the respective electrode layers are preferably bundled andwelded to a terminal. If an electrode has a large area, the internalresistance is high. Thus, multiple terminals may preferably be formed inthe electrode to reduce the resistance. In an electrode group having thewound structure, multiple lead structures may be disposed on each of thepositive electrode and the negative electrode and bundled to a terminal.Thereby, the internal resistance can be reduced.

The external case may be made of any material that is stable to anelectrolyte solution to be used. Specific examples thereof includemetals such as nickel-plated steel plates, stainless steel, aluminum andaluminum alloys, and magnesium alloys, and layered film (laminate film)of resin and aluminum foil. In order to reduce the weight, a metal suchas aluminum or an aluminum alloy or a laminate film is favorably used.

External cases made of metal may have a sealed up structure formed bywelding the metal by laser welding, resistance welding, or ultrasonicwelding or a caulking structure using the metal via a resin gasket.External cases made of a laminate film may have a sealed up structureformed by hot melting the resin layers. In order to improve thesealability, a resin which is different from the resin of the laminatefilm may be disposed between the resin layers. Especially, in the caseof forming a sealed up structure by hot melting the resin layers viacurrent collecting terminals, metal and resin are to be bonded. Thus,the resin to be disposed between the resin layers is favorably a resinhaving a polar group or a modified resin having a polar group introducedthereinto.

The lithium ion secondary battery may have any shape, and examplesthereof include cylindrical batteries, square batteries, laminatedbatteries, coin batteries, and large-size batteries. The shapes and theconfigurations of the positive electrode, the negative electrode, andthe separator may be changed in accordance with the shape of thebattery.

A module including the electrochemical device or secondary battery thatincludes the electrolyte solution of the present invention is also oneaspect of the present invention.

Examples of the electrochemical device using the electrolyte solution ofthe present invention include an electric double-layer capacitor.

In the electric double-layer capacitor, at least one of the positiveelectrode and the negative electrode is a polarizable electrode.Examples of the polarizable electrode and a non-polarizable electrodeinclude the following electrodes specifically disclosed in JP H09-7896A.

The polarizable electrode mainly containing activated carbon ispreferably one containing inactivated carbon having a large specificsurface area and a conductive material, such as carbon black, providingelectronic conductivity. The polarizable electrode can be formed by anyof various methods. For example, a polarizable electrode includingactivated carbon and carbon black can be produced by mixing activatedcarbon powder, carbon black, and phenolic resin, press-molding themixture, and then sintering and activating the mixture in an inert gasatmosphere and water vapor atmosphere. Preferably, this polarizableelectrode is bonded to a current collector using a conductive adhesive,for example.

Alternatively, a polarizable electrode can also be formed by kneadingactivated carbon powder, carbon black, and a binder in the presence ofalcohol and forming the mixture into a sheet shape, and then drying thesheet. This binder may be polytetrafluoroethylene, for example.Alternatively, a polarizable electrode integrated with a currentcollector can be produced by mixing activated carbon powder, carbonblack, a binder, and a solvent to form slurry, applying this slurry tometal foil of a current collector, and then drying the slurry.

The electric double-layer capacitor may have polarizable electrodesmainly containing activated carbon on the respective sides. Still, theelectric double-layer capacitor may have a non-polarizable electrode onone side, for example, a positive electrode mainly including anelectrode active material such as a metal oxide and a negative electrodeincluding a polarizable electrode that mainly contains activated carbonmay be combined; or a negative electrode mainly including a carbonmaterial that can reversibly occlude and release lithium ions or anegative electrode of lithium metal or lithium alloy and a polarizableelectrode mainly including activated carbon may be combined.

In place of or in combination with activated carbon, any carbonaceousmaterial such as carbon black, graphite, expanded graphite, porouscarbon, carbon nanotube, carbon nanohorn, and Kethenblack may be used.

The non-polarizable electrode is preferably an electrode mainlycontaining a carbon material that can reversibly occlude and releaselithium ions, and this carbon material is made to occlude lithium ionsin advance. In this case, the electrolyte used is a lithium salt. Theelectric double-layer capacitor having such a configuration achieves amuch higher withstand voltage of exceeding 4 V.

The solvent used in preparation of slurry in the production ofelectrodes is preferably one that dissolves a binder. In accordance withthe type of a binder, N-methylpyrrolidone, dimethyl formamide, toluene,xylene, isophorone, methyl ethyl ketone, ethyl acetate, methyl acetate,dimethyl phthalate, ethanol, methanol, butanol, or water isappropriately selected.

Examples of the activated carbon used for the polarizable electrodeinclude phenol resin-type activated carbon, coconut shell-type activatedcarbon, and petroleum coke-type activated carbon. In order to achieve alarge capacity, petroleum coke-type activated carbon or phenolresin-type activated carbon is preferably used. Examples of methods ofactivating the activated carbon include steam activation and molten KOHactivation. In order to achieve a larger capacity, activated carbonprepared by molten KOH activation is preferably used.

Preferred examples of the conductive material used for the polarizableelectrode include carbon black, Ketjenblack, acetylene black, naturalgraphite, artificial graphite, metal fiber, conductive titanium oxide,and ruthenium oxide. In order to achieve good conductivity (i.e., lowinternal resistance), and because too large an amount thereof may leadto a decreased capacity of the product, the amount of the conductivematerial such as carbon black used for the polarizable electrode ispreferably 1 to 50 mass % in the sum of the amounts of the activatedcarbon and the conductive material.

In order to provide an electric double-layer capacitor having a largecapacity and a low internal resistance, the activated carbon used forthe polarizable electrode preferably has an average particle size of 20μm or smaller and a specific surface area of 1500 to 3000 m²/g.Preferred examples of the carbon material for providing an electrodemainly containing a carbon material that can reversibly occlude andrelease lithium ions include natural graphite, artificial graphite,graphitized mesocarbon microsphere, graphitized whisker, vapor-growncarbon fiber, sintered furfuryl alcohol resin, and sintered novolakresin.

The current collector may be any chemically and electrochemicallycorrosion-resistant one. Preferred examples of the current collectorused for the polarizable electrode mainly containing activated carboninclude stainless steel, aluminum, titanium, and tantalum. Particularlypreferred materials in terms of the characteristics and cost of theresulting electric double-layer capacitor are stainless steel andaluminum. Preferred examples of the current collector used for theelectrode mainly containing a carbon material that can reversiblyocclude and release lithium ions include stainless steel, copper, andnickel.

The carbon material that can reversibly occlude and release lithium ionscan be allowed to occlude lithium ions in advance by (1) a method ofmixing powdery lithium to a carbon material that can reversibly occludeand release lithium ions, (2) a method of placing lithium foil on anelectrode including a carbon material that can reversibly occlude andrelease lithium ions and a binder so that the lithium foil iselectrically in contact with the electrode, immersing this electrode inan electrolyte solution containing a lithium salt dissolved therein sothat the lithium is ionized, and allowing the carbon material to take inthe resulting lithium ions, or (3) a method of placing an electrodeincluding a carbon material that can reversibly occlude and releaselithium ions and a binder at a minus side and placing a lithium metal ata plus side, immersing the electrodes in a non-aqueous electrolytesolution containing a lithium salt as an electrolyte, and supplying acurrent so that the carbon material is allowed to electrochemically takein the ionized lithium.

Examples of known electric double-layer capacitors include woundelectric double-layer capacitors, laminated electric double-layercapacitors, and coin-type electric double-layer capacitors. The electricdouble-layer capacitor of the present invention may also be any of thesetypes.

For example, a wound electric double-layer capacitor is assembled bywinding a positive electrode and a negative electrode each of whichincludes a laminate (electrode) of a current collector and an electrodelayer, and a separator in between to provide a wound element, puttingthis wound element in a case made of, for example, aluminum, filling thecase with an electrolyte solution, preferably a non-aqueous electrolytesolution, and sealing the case with a rubber sealant.

In the present invention, a separator formed from a conventionally knownmaterial and having a conventionally known structure can also be used.Examples thereof include polyethylene porous membranes, and nonwovenfabric of polypropylene fiber, glass fiber, or cellulose fiber.

In accordance with any known method, the capacitor may be formed into alaminated electric double-layer capacitor in which a sheet-like positiveelectrode and negative electrode are stacked with an electrolytesolution and a separator in between or a coin-type electric double-layercapacitor in which a positive electrode and a negative electrode arefixed by a gasket with an electrolyte solution and a separator inbetween.

As mentioned above, use of the electrolyte solution of the presentinvention can suitably provide secondary batteries excellent inhigh-temperature storage characteristics, modules using the secondarybatteries, and electric double-layer capacitors.

The method for producing compounds represented by the formula (1)wherein X¹¹ is O among the aforementioned compounds represented by theformula (1) is also one aspect of the present invention.

Non-Patent Literature 1 discloses that lithium monoethyl sulfate isproduced as a by-product of ionic liquid synthesis.

In order to use the lithium monoalkyl sulfate for applications aselectrolyte solutions, contamination of impurities, especially moisture,in the lithium salt needs to be avoided to the utmost so as to restrainreactions with and decomposition of a solvent and/or an electrolyte.However, since it is substantially impossible to separate lithiummonoethyl sulfate from an ionic liquid, for example, the method ofsynthesizing an ionic liquid accompanied by production of lithiummonoethyl sulfate as a by-product fails to provide lithium monoethylsulfate impurities with high purity.

The inventors have performed various studies to find that a reactionbetween a compound represented by the formula (3) and a metal alkoxidecan provide a compound having a high purity and a low moistureconcentration in high yield.

In other words, the production method of the present inventioncharacteristically includes reacting a compound represented by thefollowing formula (3) and a metal alkoxide.

The formula (3) is R³¹O—SO₂—OR³²

wherein R³¹ and R³² may be the same as or different from each other, andare individually a C1-C6 linear or branched alkyl group, a C2-C6 linearor branched alkenyl group, a C2-C6 linear or branched alkynyl group, aC3-C6 cycloalkyl group, a C3-C6 cycloalkenyl group, or a C3-C6alkylsilyl group, the alkyl group, the cycloalkyl group, and thealkylsilyl group may have a halogen atom which substitutes for ahydrogen atom bonding to a carbon atom, may have a cyclic structure, andmay have an ether bond or a thioether bond.

In the formula (3), preferably, R³¹ and R³² may be the same as ordifferent from each other, and are individually a C1-C6 linear orbranched alkyl group or a C3-C6 cycloalkyl group.

In other words, the compound represented by the formula (3) ispreferably an alkyl sulfate represented by the formula (3-1):

R³¹O—SO₂—OR³²

wherein R and R³² may be the same as or different from each other, andare individually a C1-C6 linear or branched alkyl group or a C3-C6cycloalkyl group, the alkyl group may have a halogen atom whichsubstitutes for a hydrogen atom bonding to a carbon atom, and may have acyclic structure, and the cycloalkyl group may have a halogen atom whichsubstitutes for a hydrogen atom bonding to a carbon atom.

The alkyl group or the cycloalkyl group preferably does not have ahalogen atom which substitutes for a hydrogen atom bonding to a carbonatom, does not have a cyclic structure, and does not have an ether bondor a thioether bond.

In the formula (3) and the formula (3-1), R³¹ and R³² are preferably thesame group.

In the formula (3) and the formula (3-1), R³¹ and R³² are individuallypreferably at least one selected from the group consisting of a methylgroup, an ethyl group, a propyl group, an isopropyl group, a n-butylgroup, an isobutyl group, a t-butyl group, a hexyl group, a2-fluoroethyl group, and a 2,2,2-trifluoroethyl group, more preferablyat least one selected from the group consisting of a methyl group, anethyl group, a propyl group, an isopropyl group, a n-butyl group, ahexyl group, and a 2,2,2-trifluoroethyl group, still more preferably atleast one selected from the group consisting of a methyl group, an ethylgroup, a propyl group, an isopropyl group, a n-butyl group, and a2,2,2-trifluoroethyl group, particularly preferably at least oneselected from the group consisting of a methyl group, an ethyl group, apropyl group, an isopropyl group, and a n-butyl group, further morepreferably an ethyl group.

The compound represented by the formula (3) or the compound representedby the formula (3-1) is preferably at least one selected from the groupconsisting of CH₃OSO₃CH₃, C₂H₅OSO₃C₂H₅, C₂H₅OSO₃CH₃,CH₃CH₂CH₂OSO₃CH₂CH₂CH₃, CH₃CH(CH₃)OSO₃CH(CH₃)CH₃,CH₃CH₂CH₂CH₂OSO₃CH₂CH₂CH₂CH₃, CH₃CH₂CH(CH₃)OSO₃CH(CH₃)CH₂CH₃,CH₃C(CH₃)₂OSO₃C(CH₃)₂CH₃, CFH₂CH₂OSO₃CH₂CFH₂, and CF₃CH₂OSO₃CH₂CF₃, morepreferably at least one selected from the group consisting ofCH₃OSO₃CH₃, C₂H₅OSO₃C₂H₅, CH₃CH₂CH₂OSO₃CH₂CH₂CH₃,CH₃CH(CH₃)OSO₃CH(CH₃)CH_(3r) and CH₃CH₂CH₂CH₂OSO₃CH₂CH₂CH₂CH₃, stillmore preferably C₂H₅OS O₃C₂H₅.

The metal alkoxide is preferably a lithium alkoxide. The lithiumalkoxide is preferably represented by the formula: R⁴—OLi, where R⁴ is aC1-C6 linear or branched alkyl group or a C3-C6 cycloalkyl group, thealkyl group may have a halogen atom which substitutes for a hydrogenatom bonding to a carbon atom, and may have a cyclic structure, and thecycloalkyl group may have a halogen atom which substitutes for ahydrogen atom bonding to a carbon atom.

R⁴ is preferably at least one selected from the group consisting of amethyl group, an ethyl group, a propyl group, an isopropyl group, and abutyl group, more preferably at least one selected from the groupconsisting of a methyl group and an ethyl group.

Specific examples of the lithium alkoxide include lithium methoxide,lithium ethoxide, lithium isopropoxide, and lithium butoxide. Inconsideration of easy availability, high reactivity, and a low boilingpoint of an ether compound which is a reaction by-product, the lithiumalkoxide is preferably at least one selected from the group consistingof lithium methoxide and lithium ethoxide.

The metal alkoxide may be used in the form of a solution in alcohol, ormay be one generated in a reactor through a known method of reacting analcohol, lithium metal, and other components such as hydrogenatedlithium.

The production method of the present invention preferably furtherincludes reacting an alcohol and lithium metal to provide a lithiumalkoxide.

The reaction is preferably caused by adding 0.7 to 1.5 mol, morepreferably 1.0 to 1.1 mol, of the metal alkoxide relative to 1 mol ofthe compound represented by the formula (3). Too small an amount of themetal alkoxide may cause a large amount of the compound represented bythe formula (3) with a high boiling point to remain unreacted. Thus,evaporation at high temperature or washing with a solvent is needed,possibly complicating the procedure. Too large an amount of the metalalkoxide may cause a further reaction between the generated compoundrepresented by the formula (1) and the metal alkoxide to generatelithium sulfate, possibly resulting in a low purity of the compoundrepresented by the formula (1). Further, the metal alkoxide may remainunreacted, possibly affecting the battery characteristics.

The reaction between the compound represented by the formula (3) and themetal alkoxide is preferably performed at −10° C. to 80° C., morepreferably 10° C. to 50° C. Too low a reaction temperature may cause aslow reaction rate, possibly failing to complete the reaction. Too higha reaction temperature may cause progress of the reaction between theresulting compound represented by the formula (1) and the metal alkoxideto generate lithium sulfate, possibly lowering the purity of thecompound represented by the formula (1).

Examples of a method of bringing the compound represented by the formula(3) and the metal alkoxide into contact with each other include a methodof adding dropwise an alcoholic solution of the metal alkoxide to thecompound represented by the formula (3) adjusted to a predeterminedtemperature; a method of adding the metal alkoxide in the form of solidto the compound represented by the formula (3) adjusted to apredetermined temperature; and a method of adding dropwise the compoundrepresented by the formula (3) to an alcoholic solution of the metalalkoxide adjusted to a predetermined temperature. For the reasons suchas easy handleability of a device, easiness of the operation, andrestraint of the reaction between the resulting compound represented bythe formula (1) and the metal alkoxide, preferred is a method of addingdropwise an alcoholic solution of the metal alkoxide to the compoundrepresented by the formula (3).

The resulting compound represented by the formula (1) can be separatedand purified by known methods. For example, washing with a solvent,extraction, recrystallization, and others may be applied.

The purity of the resulting compound represented by the formula (1) maybe quantified by the internal standard method utilizing ¹H-NMR analysis.The sulfate ion concentration in the compound represented by the formula(1) may be quantified by the external standard method utilizing ionchromatograph (IC) analysis. The moisture concentration in the lithiumsalt may be determined by, for example, the distillation method, thepotentiometric Karl Fischer titration method, the coulometric KarlFischer titration method, or the hydride reaction method, prescribed inJIS K2275. The potentiometric Karl Fischer titration method and thecoulometric Karl Fischer titration method can employ a commerciallyavailable Karl Fischer moisture meter.

Since the compound represented by the formula (1) obtained by theproduction method of the present invention has a high purity, it can besuitably used as an additive for electrolyte solutions. Use of anelectrolyte solution containing the compound represented by the formula(1) obtained by the production method of the present invention enablesproduction of electrochemical devices whose internal resistance is lesslikely to increase even after repeated charge and discharge and whosecycle capacity retention ratio is high.

The electrolyte solution preferably contains a solvent, an electrolytesalt, and the compound represented by the formula (1) obtained by theproduction method of the present invention. Preferred types and amountsof the respective components can be those described for the electrolytesolution of the present invention.

EXAMPLES

The present invention will be described with reference to, but notlimited to, examples.

(Method for Producing Lithium Monoalkyl Sulfate) Synthesis Example 1:Production of Lithium Monomethyl Sulfate

A 2-L glass reactor was charged with 400 g (3.2 mol) of dimethylsulfate, and the temperature was controlled by water bath such that theinternal temperature was 25° C. A dropping funnel was charged with 1.23kg (3.2 mol) of a 10 mass % lithium methoxide/methanol solution, and thesolution was added dropwise to the system such that the internaltemperature was 40° C. or lower. The reaction solution was slightlycloudy immediately after the dropwise addition. The reaction solutionwas then stirred for one hour at the same temperature, resulting inremoval of the cloudiness.

The resulting reaction solution was analyzed by gas chromatography (GC),and no peak derived from the material, i.e., dimethyl sulfate, wasobserved.

Components such as methanol and dimethyl ether which is a by-product,occupying most part of the reaction solution, were distilled off at areduced pressure of 200 Pa and an internal temperature of 60° C., andthen the above compounds and others were further distilled off at areduced pressure of 30 Pa and an internal temperature of 150° C.Thereby, 370 g of white solid was obtained.

The purity of lithium monomethyl sulfate calculated by the internalstandard method utilizing ¹H-NMR was 99 mass % or more, and the sulfateion concentration calculated by the external standard method utilizingion chromatograph was 6200 mass ppm. The moisture concentrationcalculated by the coulometric Karl Fischer titration method was 103 massppm.

Synthesis Example 2: Production of Lithium Monoethyl Sulfate

A 2-L glass reactor was charged with 500 g (3.2 mol) of diethyl sulfate,and the temperature was controlled by water bath such that the internaltemperature was 25° C. A dropping funnel was charged with 1.26 kg (3.3mol) of a 10 mass % lithium methoxide/methanol solution, and thesolution was added dropwise to the system such that the internaltemperature was 40° C. or lower. The reaction solution was slightlycloudy immediately after the dropwise addition. The reaction solutionwas then stirred for five hours at the same temperature, resulting inremoval of the cloudiness.

The resulting reaction solution was analyzed by gas chromatography (GC),and no peak derived from the material, i.e., diethyl sulfate, wasobserved.

Components such as methanol and ethyl methyl ether which is a by-productin the reaction solution were distilled off at a reduced pressure of 200Pa and an internal temperature of 60° C., and then the above compoundsand others were further distilled off at a reduced pressure of 30 Pa andan internal temperature of 150° C. Thereby, 420 g of white solid wasobtained.

The purity of lithium monoethyl sulfate calculated by the internalstandard method utilizing ¹H-NMR was 99 mass % or more, and the sulfateion concentration calculated by the external standard method utilizingion chromatograph was 2 mass ppm. The moisture concentration calculatedby the coulometric Karl Fischer titration method was 148 mass ppm.

Synthesis Example 3: Production of Lithium Mono-n-Butyl Sulfate

A 200-mL glass reactor was charged with 50 g (0.24 mol) of di-n-butylsulfate, and the temperature was controlled by water bath such that theinternal temperature was 25° C. A dropping funnel was charged with 95 g(0.25 mol) of a 10 mass % lithium methoxide/methanol solution, and thesolution was added dropwise to the system such that the internaltemperature was 40° C. or lower. Immediately after the dropwiseaddition, the reaction solution was heated up to 40° C., and thenstirred for five hours at the same temperature.

The resulting reaction solution was analyzed by gas chromatography (GC),and no peak derived from the material, i.e., di-n-butyl sulfate, wasobserved.

Components such as methanol and butyl methyl ether which is a by-productin the reaction solution were distilled off at a reduced pressure of 200Pa and an internal temperature of 60° C., and then the above compoundsand others were further distilled off at a reduced pressure of 30 Pa andan internal temperature of 150° C. Thereby, 35 g of white solid wasobtained.

The purity of lithium mono-n-butyl sulfate calculated by the internalstandard method utilizing ¹H-NMR was 98 mass % or more, and the sulfateion concentration calculated by the external standard method utilizingion chromatograph was 25 mass ppm. The moisture concentration calculatedby the coulometric Karl Fischer titration method was 170 mass ppm.

Comparative Synthesis Example 1

A 300-mL glass reactor was charged with 50 g (0.32 mol) of diethylsulfate, and the temperature was controlled by water bath such that theinternal temperature was 25° C. Then, 126 g (3.9 mol) of methanol wasadded dropwise to the system through a dropping funnel, and nogeneration of heat was observed. The system was then stirred at the sametemperature for five hours.

The resulting reaction solution was analyzed by gas chromatography (GC),and diethyl sulfate and methanol were detected at the peak ratiocalculated from the ratio between the amounts thereof prepared. Thisconfirmed no progress of the reaction.

Experiment 1 (Evaluation of 4.4 V Grade Lithium Battery)

Electrolyte solutions of Examples 1 to 21 and electrolyte solutions ofComparative Examples 1 to 6 were prepared as follows and lithium ionsecondary batteries were produced using the resulting electrolytesolutions. The resistance increasing rates and the cycle capacityretention ratios of the respective batteries were evaluated.

(Preparation of Electrolyte Solution)

An acyclic carbonate(s) and a cyclic carbonate(s) were mixed in a ratioshown in Table 1 under dry argon atmosphere. To this solution was addeddry lithium monoalkyl sulfate in an amount shown in Table 1, and dryLiPF₆ was further added so as to be a concentration of 1.0 mol/L.Thereby, a non-aqueous electrolyte solution was obtained. The amounts ofthe additives such as lithium monoalkyl sulfate blended were expressedby mass % relative to the acyclic carbonate(s) and the cycliccarbonate(s).

The compounds in the tables are as follows.

Acyclic Carbonates

a: dimethyl carbonate

b: ethylmethyl carbonate

c: diethyl carbonate

d: CF₃CH₂OCOOCH₃

e: CF₃CH₂OCOOCH₂CF₃

Cyclic Carbonates

EC: ethylene carbonate

FEC: 4-fluoro-1,3-dioxolan-2-one

Additives

F: C₂H₅OSO₃Li

G: CH₃CH₂CH₂OSO₃Li

H: CH₃CH₂CH₂CH₂OSO₃Li

I: CH₃ (CH₂)₁₁OSO₃Na

J: (CH₃)₂NSO₃Li

K: (C₂H₅)₂NSO₃Li

(Production of Negative Electrode)

Powder of artificial graphite used as a negative electrode activematerial, an aqueous dispersion of carboxymethyl cellulose sodium(concentration of carboxymethyl cellulose sodium: 1 mass %) used as athickening agent, and an aqueous dispersion of styrene-butadiene rubber(concentration of styrene-butadiene rubber: 50 mass %) used as a binderwere mixed in a water solvent to prepare a negative electrode mixtureslurry. The solid content ratio of the negative electrode activematerial, the thickening agent, and the binder was 97.6/1.2/1.2 (mass %ratio). The slurry was uniformly applied to 20-μm-thick copper foil,followed by drying, and then the workpiece was compression-molded with apress. Thereby, a negative electrode was prepared.

(Production of Positive Electrode)

LiCoO₂ used as a positive electrode active material, acetylene blackused as a conductive material, and a dispersion of polyvinylidenefluoride (PVdF) in N-methyl-2-pyrrolidone used as a binder were mixed toprepare a positive electrode mixture slurry. The solid content ratio ofthe positive electrode active material, the conductive material, and thebinder was 92/3/5 (mass % ratio). The positive electrode mixture slurrywas uniformly applied to a 20-μm-thick current collector made ofaluminum foil, followed by drying, and then the workpiece wascompression-molded with a press. Thereby, a positive electrode wasprepared.

(Production of Lithium Ion Secondary Battery)

The above prepared negative electrode, a polyethylene separator, and theabove prepared positive electrode were stacked in the given order toprovide a battery element.

A bag made of a laminate film in which an aluminum sheet (thickness: 40μm) was coated with a resin layer on each side was prepared. The abovebattery element was placed in the bag in such a manner that theterminals of the positive electrode and negative electrode stuck out ofthe bag. One of the electrolyte solutions of Examples 1 to 21 andComparative Examples 1 to 6 was poured into the bag and the bag wasvacuum sealed. Thereby, a lithium ion secondary battery in a sheet formwas produced.

<High-Temperature Cycle Capacity Retention Ratio>

The above produced secondary battery in the state of being sandwichedand pressurized between plates was subjected to constantcurrent-constant voltage charge (hereinafter, referred to as CC/CVcharge) (0.1 C cut off) to 4.4 V at a current corresponding to 0.2 C at60° C. Then, the battery was discharged to 3 V at a constant current of0.2 C. This process was counted as one cycle. The initial dischargecapacity was determined from the discharge capacity of the third cycle.Here, 1 C means a current value required for discharging the referencecapacity of a battery in an hour. For example, 0.2 C indicates a ⅕current value thereof. The cycle was again repeated, and the dischargecapacity after 100 cycles was defined as the capacity after cycles. Theratio of the discharge capacity after 100 cycles to the initialdischarge capacity was determined, which was regarded as a cyclecapacity retention ratio (%).

(Discharge capacity after 100 cycles)/(initial dischargecapacity)×100=cycle capacity retention ratio (%)

<Resistance Increasing Rate>

A charge and discharge cycle under predetermined charge and dischargeconditions (charge at 0.2 C and a predetermined voltage until the chargecurrent reached 1/10 C, and discharge at a current corresponding to 1 Cto 3.0 V) was defined as one cycle. The resistance after three cyclesand the resistance after 100 cycles were determined. The measurementtemperature was 25° C. The resistance increasing rate after 100 cycleswas determined by the following formula.

Table 1 shows the results.

Resistance increasing rate (%)=(resistance (Ω) after 100cycles)/(resistance (Ω) after three cycles)×100

TABLE 1 Components constituting electrolyte solution Acyclic carbonateCyclic carbonate Additive Cycle capacity Resistance Mixing ratio Mixingratio Mixing ratio retention ratio increasing Structure (vol %)Structure (vol %) Structure (mass %) (%) rate (%) Example 1 Component(b) 70 EC 30 Component (F) 2.0 93 134 Example 2 Component (b) 70 EC 30Component (F) 0.001 88 153 Example 3 Component (b) 70 EC 30 Component(F) 0.01 90 148 Example 4 Component (b) 70 EC 30 Component (F) 0.1 91142 Example 5 Component (b) 70 EC 30 Component (F) 0.5 92 138 Example 6Component (b) 70 EC 30 Component (F) 1.0 92 137 Example 7 Component (b)70 EC 30 Component (F) 5.0 91 132 Example 8 Component (b) 70 EC 30Component (F) 10.0 90 131 Example 9 Component (a) 70 EC 30 Component (F)2.0 90 153 Example 10 Component (c) 70 EC 30 Component (F) 3.0 91 141Example 11 Component (a) 70 FEC 30 Component (F) 2.0 90 144 Example 12Component (b) 70 FEC 30 Component (F) 2.5 92 141 Example 13 Component(c) 70 FEC 30 Component (F) 2.5 92 139 Example 14 Component (a) + 25 +45 EC 30 Component (F) 1.0 91 145 Component (b) Example 15 Component (b)70 EC + FEC 20 + 10 Component (F) 2.0 91 141 Example 16 Component (b) 50EC 50 Component (F) 2.0 88 137 Example 17 Component (b) 70 EC 30Component (G) 2.0 90 140 Example 18 Component (b) 80 EC + FEC 15 + 5 Component (F) + 2.0 + 2.0 91 137 Component (G) Example 19 Component (b)70 EC 30 Component (H) 2.0 88 142 Example 20 Component (b) 70 EC 30Component (J) 0.5 90 145 Example 21 Component (b) 70 EC 30 Component (K)0.5 91 142 Comparative Example 1 Component (b) 70 EC 30 — 0 87 165Comparative Example 2 Component (b) 70 EC 30 Component (F) 13.0 72 194Comparative Example 3 Component (b) 70 EC 30 Component (H) 15.0 64 187Comparative Example 4 Component (b) 60 EC + FEC 20 + 20 Component (G) +8.0 + 8.0 68 178 Component (H) Comparative Example 5 Component (b) 60FEC 40 — 0 88 170 Comparative Example 6 Component (b) 70 EC 30 Component(I) 2.0 84 180

The table shows that the lithium ion secondary batteries produced usingthe electrolyte solutions of Examples 1 to 21 had a lower resistanceincreasing rate than the lithium ion secondary batteries produced usingthe electrolyte solutions of Comparative Examples 1 to 6.

Experiment 2 (Evaluation of 4.9 V Grade Lithium Battery)

Electrolyte solutions of Examples 22 to 41 and electrolyte solutions ofComparative Examples 7 to 15 were prepared as follows and lithium ionsecondary batteries were produced using the resulting electrolytesolutions. The resistance increasing rates and the cycle capacityretention ratios of the respective batteries were evaluated.

(Preparation of Electrolyte Solution)

An acyclic carbonate(s) and a cyclic carbonate(s) were mixed in a ratioshown in Table 2 under dry argon atmosphere. To this solution was addeddry lithium monoalkyl sulfate in an amount shown in Table 2, and dryLiPF₆ was further added so as to be a concentration of 1.0 mol/L.Thereby, a non-aqueous electrolyte solution was obtained. The amounts ofthe additives such as lithium monoalkyl sulfate blended were expressedby mass % relative to the acyclic carbonate(s) and the cycliccarbonate(s).

The components shown in Table 2 are the same as those in Table 1.

(Production of Negative Electrode)

Powder of artificial graphite used as a negative electrode activematerial, an aqueous dispersion of carboxymethyl cellulose sodium(concentration of carboxymethyl cellulose sodium: 1 mass %) used as athickening agent, and an aqueous dispersion of styrene-butadiene rubber(concentration of styrene-butadiene rubber: 50 mass %) used as a binderwere mixed in a water solvent to prepare a negative electrode mixtureslurry. The solid content ratio of the negative electrode activematerial, the thickening agent, and the binder was 97.6/1.2/1.2 (mass %ratio). The slurry was uniformly applied to 20-μm-thick copper foil,followed by drying, and then the workpiece was compression-molded with apress. Thereby, a negative electrode was prepared.

(Production of Positive Electrode)

LiNi_(0.5)Mn_(1.5)O₄ used as a positive electrode active material,acetylene black used as a conductive material, and a dispersion ofpolyvinylidene fluoride (PVdF) in N-methyl-2-pyrrolidone used as abinder were mixed to prepare a positive electrode mixture slurry. Thesolid content ratio of the positive electrode active material, theconductive material, and the binder was 92/3/5 (mass % ratio). Thepositive electrode mixture slurry was uniformly applied to a 20-μm-thickcurrent collector made of aluminum foil, followed by drying, and thenthe workpiece was compression-molded with a press. Thereby, a positiveelectrode was prepared.

(Production of Lithium Ion Secondary Battery)

The above prepared negative electrode, a polyethylene separator, and theabove prepared positive electrode were stacked in the given order toprovide a battery element.

A bag made of a laminate film in which an aluminum sheet (thickness: 40μm) was coated with a resin layer on each side was prepared. The abovebattery element was placed in the bag in such a manner that theterminals of the positive electrode and negative electrode stuck out ofthe bag. One of the electrolyte solutions of Examples 22 to 41 andComparative Examples 7 to 15 was poured into the bag and the bag wasvacuum sealed. Thereby, a lithium ion secondary battery in a sheet formwas produced.

<High-Temperature Cycle Capacity Retention Ratio>

The above produced secondary battery in the state of being sandwichedand pressurized between plates was subjected to constantcurrent-constant voltage charge (hereinafter, referred to as CC/CVcharge) (0.1 C cut off) to 4.9 V at a current corresponding to 0.2 C at60° C. Then, the battery was discharged to 3 V at a constant current of0.2 C. This process was counted as one cycle. The initial dischargecapacity was determined from the discharge capacity of the third cycle.Here, 1 C means a current value required for discharging the referencecapacity of a battery in an hour. For example, 0.2 C indicates a ⅕current value thereof. The cycle was again repeated, and the dischargecapacity after 100 cycles was defined as the capacity after cycles. Theratio of the discharge capacity after 100 cycles to the initialdischarge capacity was determined, which was regarded as a cyclecapacity retention ratio (%).

(Discharge capacity after 100 cycles)/(initial dischargecapacity)×100=cycle capacity retention ratio (%)

<Resistance Increasing Rate>

A charge and discharge cycle under predetermined charge and dischargeconditions (charge at 0.2 C and a predetermined voltage until the chargecurrent reached 1/10 C, and discharge at a current corresponding to 1 Cto 3.0 V) was defined as one cycle. The resistance after three cyclesand the resistance after 100 cycles were determined. The measurementtemperature was 25° C. The resistance increasing rate after 100 cycleswas determined by the following formula.

Resistance increasing rate (%) after 100 cycles=(resistance (Ω) after100 cycles)/(resistance (Ω) after three cycles)×100

TABLE 2 Components constituting electrolyte solution Resistance Acycliccarbonate Cyclic carbonate Additive Cycle capacity increasing Mixingratio Mixing ratio Mixing ratio retention ratio rate Structure (vol %)Structure (vol %) Structure (mass %) (%) (%) Example 22 Component (d) 60FEC 40 Component (F) 2.0 89 150 Example 23 Component (d) 60 FEC 40Component (F) 0.001 83 158 Example 24 Component (d) 60 FEC 40 Component(F) 0.01 85 154 Example 25 Component (d) 60 FEC 40 Component (F) 0.1 86152 Example 26 Component (d) 60 FEC 40 Component (F) 0.5 87 151 Example27 Component (d) 60 FEC 40 Component (F) 1.0 87 151 Example 28 Component(d) 60 FEC 40 Component (F) 5.0 86 146 Example 29 Component (d) 60 FEC40 Component (F) 10.0 85 147 Example 30 Component (e) 60 FEC 40Component (F) 2.0 85 150 Example 31 Component (d) + Component (e) 30 +30 FEC 40 Component (F) 3.0 86 150 Example 32 Component (d) + Component(e) 55 + 5  EC + FEC 10 + 30 Component (F) 4.0 87 155 Example 33Component (b) + Component (d) 10 + 60 FEC 30 Component (F) 2.0 85 156Example 34 Component (b) + Component (d) 15 + 35 FEC 50 Component (F)2.5 83 154 Example 35 Component (b) + Component (e) 10 + 60 FEC 30Component (F) 2.5 87 154 Example 36 Component (a) + Component (b) 25 +45 EC 30 Component (F) 1.0 86 156 Example 37 Component (d) 70 FEC 30Component (G) 2.0 85 161 Example 38 Component (d) 80 EC + FEC  5 + 15Component (F) + 2.0 + 2.0 86 150 Component (G) Example 39 Component (d)70 FEC 30 Component (H) 2.0 83 156 Example 40 Component (d) 60 FEC 40Component (J) 0.5 82 158 Example 41 Component (d) 60 FEC 40 Component(K) 0.5 86 156 Comparative Component (b) 70 EC 30 — 0 40 218 Example 7Comparative Component (d) 60 FEC 40 — 0 79 160 Example 8 ComparativeComponent (d) 70 FEC 30 Component (H) 18.0 59 190 Example 9 ComparativeComponent (d) + Component (e) 55 + 5  FEC 40 Component (F) 20.0 71 174Example 10 Comparative Component (b) 60 EC + FEC 20 + 20 Component (G) +8.0 + 8.0 63 183 Example 11 Component (H) Comparative Component (b) 60FEC 40 — 0 84 174 Example 12 Comparative Component (b) 70 EC 30Component (I) 2.0 38 224 Example 13 Comparative Component (d) 70 FEC 30Component (F) 13.0 77 180 Example 14 Comparative Component (d) 70 FEC 30Component (F) 15.0 75 185 Example 15

The table shows that the lithium ion secondary batteries produced usingthe electrolyte solutions of Examples 22 to 41 had a lower resistanceincreasing rate than the lithium ion secondary batteries produced usingthe electrolyte solutions of Comparative Examples 7 to 15.

INDUSTRIAL APPLICABILITY

The electrolyte solution of the present invention can be suitably usedas an electrolyte solution for electrochemical devices such as lithiumion secondary batteries.

1. An electrolyte solution comprising: a solvent; an electrolyte salt;and at least one selected from the group consisting of compoundsrepresented by the following formula (1) and compounds represented bythe following formula (2) in an amount of 0.001 to 10 mass % relative tothe solvent, the solvent containing a fluorinated acyclic carbonate,the formula (1) being R¹¹X¹¹—SO₃M¹¹ wherein R¹¹ is a C1-C6 linear orbranched alkyl group, a C2-C6 linear or branched alkenyl group, a C2-C6linear or branched alkynyl group, a C3-C6 cycloalkyl group, a C3-C6cycloalkenyl group, or a C3-C6 alkylsilyl group, the alkyl group, thecycloalkyl group, and the alkylsilyl group may have a halogen atom whichsubstitutes for a hydrogen atom bonding to a carbon atom, may have acyclic structure, and may have an ether bond or a thioether bond; X¹¹ isO or S; and M¹¹ is at least one selected from the group consisting ofLi, Na, K, and Cs, andthe formula (2) being R²¹R²²N—SO₃M²¹ wherein R²¹ and R²² may be the sameas or different from each other, and are individually a C1-C6 linear orbranched alkyl group, a C2-C6 linear or branched alkenyl group, a C2-C6linear or branched alkynyl group, a C3-C6 cycloalkyl group, a C3-C6cycloalkenyl group, or a C3-C6 alkylsilyl group, the alkyl group, thecycloalkyl group, and the alkylsilyl group may have a halogen atom whichsubstitutes for a hydrogen atom bonding to a carbon atom, may have acyclic structure, and R¹¹ and R²² may bond to each other to form acyclic structure; and M²¹ is at least one selected from the groupconsisting of Li, Na, K, and Cs.
 2. The electrolyte solution accordingto claim 1, wherein, in the formula (1), R¹¹ is a C1-C6 linear orbranched alkyl group or a C3-C6 cycloalkyl group; X¹¹ is O; and M¹¹ isLi.
 3. The electrolyte solution according to claim 1, wherein, in theformula (2), R²¹ and R²² may be the same as or different from eachother, and are individually a C1-C6 linear or branched alkyl group or aC3-C6 cycloalkyl group; and M²¹ is Li.
 4. (canceled)
 5. The electrolytesolution according to claim 1, wherein the electrolyte salt is at leastone lithium salt selected from the group consisting of LiPF₆, LiBF₄,LiSO₃CF₃, LiN(SO₂CF₃)₂, LiN(SO₂C₂F₅)₂, lithium difluoro(oxalato)borate,lithium bis(oxalato)borate, and salts represented byLiPF_(a)(C_(n)F_(2n+1))_(6−a), where a is an integer of 0 to 5; and n isan integer of 1 to
 6. 6. An electrochemical device comprising theelectrolyte solution according to claim
 1. 7. A secondary batterycomprising the electrolyte solution according to claim
 1. 8. A modulecomprising the electrochemical device according to claim
 6. 9. A methodfor producing a compound represented by the following formula (1),comprising: reacting a compound represented by the following formula (3)and a metal alkoxide,the formula (1) being R¹¹X¹¹—SO₃M¹¹ wherein R¹¹ is a C1-C6 linear orbranched alkyl group, a C2-C6 linear or branched alkenyl group, a C2-C6linear or branched alkynyl group, a C3-C6 cycloalkyl group, a C3-C6cycloalkenyl group, or a C3-C6 alkylsilyl group, the alkyl group, thecycloalkyl group, and the alkylsilyl group may have a halogen atom whichsubstitutes for a hydrogen atom bonding to a carbon atom, may have acyclic structure, and may have an ether bond or a thioether bond; X¹¹ isO; and M¹¹ is at least one selected from the group consisting of Li, Na,K, and Cs, andthe formula (3) being R³¹O—SO₂—OR³² wherein R³¹ and R³² may be the sameas or different from each other, and are individually a C1-C6 linear orbranched alkyl group, a C2-C6 linear or branched alkenyl group, a C2-C6linear or branched alkynyl group, a C3-C6 cycloalkyl group, a C3-C6cycloalkenyl group, or a C3-C6 alkylsilyl group, the alkyl group, thecycloalkyl group, and the alkylsilyl group may have a halogen atom whichsubstitutes for a hydrogen atom bonding to a carbon atom, may have acyclic structure, and may have an ether bond or a thioether bond. 10.The production method according to claim 9, wherein, in the formula (1),R¹¹ is a C1-C6 linear or branched alkyl group or a C3-C6 cycloalkylgroup; and M¹¹ is Li.
 11. The production method according to claim 9,wherein, in the formula (3), R³¹ and R³² may be the same as or differentfrom each other, and are individually a C1-C6 linear or branched alkylgroup or a C3-C6 cycloalkyl group.
 12. The production method accordingto claim 9, wherein the metal alkoxide is a lithium alkoxide.
 13. Theproduction method according to claim 9, wherein the metal alkoxide is atleast one selected from the group consisting of lithium methoxide andlithium ethoxide.
 14. The production method according to claim 12,further comprising reacting an alcohol with lithium metal to provide alithium alkoxide.
 15. The production method according to claim 9,wherein the reaction is caused by adding 0.7 to 1.5 mol of the metalalkoxide relative to 1 mol of the compound represented by the formula(3).
 16. A module comprising the secondary battery according to claim 7.